Run unit tests on macOS in CI

[twz123]

Description

• Compile UnmountLazy just for Linux. MNT_DETACH is defined for Linux only, hence change the build tag from unix to linux and provide a fallback for everything non-Linux.
• Provide winExecute fallback not only for Linux. This is necessary to make it compile on macOS.
• Make unit tests pass on macOS. Skip one test that times out for unknown reasons and needs further debugging.
• Combine the linux and windows unit test jobs into one matrix job and add a new matrix entry for macOS.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings