Fix static integrity classes tests in Windows

[barbazul]

Description

This changes how the path for the file ClassesTest is evaluated so it is correctly skipped in windows

Fixed Issues (if relevant)

1. Fixes Static tests in Windows fail due to file path mismatches #9805 Static tests in Windows fail due to file path mismatches

Manual testing scenarios

1. cd [Magento path]\dev\tests\static
2. php ......\vendor\phpunit\phpunit\phpunit --filter=ClassesTest

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

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