Rename shared example file to fix RSpec warning

If the name of a file under spec/support ends with `_spec.rb`, it will
be loaded first by the RSpec main command and then by `spec_helper.rb`,
causing an RSpec warning:

```
WARNING: Shared example group 'a criterion' has been previously defined
at:
  /home/su/code/Markus/spec/support/criterion_spec.rb:1
...and you are now defining it at:
  /home/su/code/Markus/spec/support/criterion_spec.rb:1
The new definition will overwrite the original one.
```

This patch fixes it by removing the `_spec` suffix. future Shared
example files should follow this rule.

Tested:
- rspec
Merge branch 'rspec_warning' of https://github.com/zhangsu/Markus into rspec_warning