Added check for duplicated file names

[davidperezgar]

Fixes #710

[davidperezgar]

@swissspidy this is the part of badly names that suggested @ernilambar to move it away, so we can work on reflection tests.

[ernilambar]

@davidperezgar I have updated the test for the check. But I noticed current check does not account for the duplicated folder name. Example:

UNIT_TESTS_PLUGIN_DIR . 'test-plugin-file-type-badly-named-files-errors/sub directory/file-1.php'
UNIT_TESTS_PLUGIN_DIR . 'test-plugin-file-type-badly-named-files-errors/Sub Directory/file-2.php'

I think we should also check for folder. We should refine the check along with error code - "duplicated_files"

[davidperezgar]

Hey awesome! It's ready then to review!

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