refactor(tests/frontends/lean): use consistent name convention for file names

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lean/scanner): add more tests for improving coverage

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lean/parser): add more tests for improving coverage

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>