Our Windows builder frequently leaves directories behind after the Tribler tests have finished

[devos50]

Our tests are writing files to the /tmp directory and are not properly removed, at least on our Windows 64-bit machine that run the tests. See the following screenshot:

There seem to be two kind of directories that are left behind:

• Directories named with ~10 random characters: they contain the following items:
  
  This makes me suspect that these directories are left behind by a test that does something with downloading (e.g., our lib torrent or channel tests).
• Directories that contain libtorrent metadata when doing DHT lookups. The directory is created here and should be cleaned-up here.

[qstokkink]

For the second bullet point:

tribler/src/tribler-core/tribler_core/components/libtorrent/download_manager/download_manager.py

Line 143 in bd86c5e

self.metadata_tmpdir = Path.mkdtemp(suffix='tribler_metainfo_tmpdir')

This temporary directory should be managed by the test suite. At least during the tests, this temporary directory should be created by pytest. Otherwise, should anything go wrong, nothing is managing the directory.