Make sure @name_stack is defined

When @name_stack is not defined, it warns by [this commit](https://github.com/elixir-lang/elixir/commit/9904bdce9d5f77de2f53a0033cb9ff930f4a3432).

before
```
/Users/yuki_ito/work/elixir/amrita% mix test
Compiled lib/amrita/elixir/string.ex
Compiled lib/amrita/elixir/pipeline.ex
Compiled lib/amrita/elixir/set.ex
Compiled lib/amrita/fact_error.ex
/Users/yuki_ito/work/elixir/amrita/lib/amrita/mocks/history.ex:6: variable a is unused
Compiled lib/amrita/mocks/history.ex
Compiled lib/amrita/mocks.ex
Compiled lib/amrita/formatter/progress.ex
Compiled lib/amrita.ex
Generated amrita.app
/Users/yuki_ito/work/elixir/amrita/test/unit/amrita/fact_error_test.exs:6: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/unit/amrita/mocks/history_test.exs:8: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:19: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:24: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:30: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:36: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:41: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:55: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:69: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:75: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
Future fact: mock with a elixir complex argument
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:89: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:95: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:102: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:108: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:114: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:120: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:126: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
/Users/yuki_ito/work/elixir/amrita/test/integration/mocks_test.exs:132: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
Future fact: regex are matched against argument parameters
/Users/yuki_ito/work/elixir/amrita/test/integration/amrita_test.exs:8: (module) undefined module attribute @name_stack, please remove access to @name_stack or explicitly set it to nil before access
Future fact: about collection matchers: without a body is considered pending
..............................................

Finished in 0.8 seconds (0.6s on load, 0.2s on tests)
46 facts, 0 failures
```

after
```
/Users/yuki_ito/work/elixir/amrita% mix test
Compiled lib/amrita/elixir/string.ex
Compiled lib/amrita/elixir/pipeline.ex
Compiled lib/amrita/elixir/set.ex
Compiled lib/amrita/fact_error.ex
/Users/yuki_ito/work/elixir/amrita/lib/amrita/mocks/history.ex:6: variable a is unused
Compiled lib/amrita/mocks/history.ex
Compiled lib/amrita/mocks.ex
Compiled lib/amrita/formatter/progress.ex
Compiled lib/amrita.ex
Generated amrita.app
Future fact: mock with a elixir complex argument
Future fact: regex are matched against argument parameters
Future fact: about collection matchers: without a body is considered pending
..............................................

Finished in 0.7 seconds (0.5s on load, 0.2s on tests)
46 facts, 0 failures
```