Tests that no Windows handles are leaked in Directory APIs

[Squareys]

Hi @mosra !

As per gitter, here's the fix for the leaking handle. Windows will refuse to delete folders with open handles, which means every list()-ed folder would be locked until application exit.

Best,
Jonathan

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Edit by @mosra -- things to do in addition to Windows handle leak counting because yes everything turns into a "fixing a lightbulb" problem after a while:

• Make it possible to have checks in a teardown routine (I have branch for this somewhere)
• Turn this into a teardown check that's attached for test cases that access the OS APIs
• Figure out a way to check file descriptor leaks on Unix as well (mainly for tests that map empty files, etc.)
  • On Linux check the number of entries in /proc/self/fd before and after
  • On macOS it's /dev/fd?? https://stackoverflow.com/a/42483033
• Checking that we're errno clean in Directory APIs / tests (in particular, if some API such as rm() fails, it should clean errno after itself)

[mosra]

Wow, this is cool, didn't even know such thing was possible.

You know what? This would be good to do for everything -- I think this could be doable in setup/teardown routines, but I need to patch TestSuite a bit to allow checks there. Is it okay if I merge just the first part with the scope guard so you have the fix and leave this extra check for later when TestSuite is ready?

[Squareys]

We can also merge this and refactor it once test suite is ready (or refactor with making it ready).

[mosra]

Merged the fix alone in 515cc7e, keeping this open so I have a reminder to add the handle counting to all tests.

Thanks!