Investigate flakiness of marked mountpoint tests

[vxgmichel]

Following up on #811, #1036, #1131, #1225, #1230, #1228, Scille/winfspy#24.

Now that #1228 is merged, four mountpoint tests are marked as flaky:

• test_mountpoint_revoke_access
• test_folder_operations
• test_winfstests
• test_winfsp_tests (although there's been no occurrences of flakiness for this one)

The issue for at least some of them seems to be related to windows deletion not being performed correctly (see the analysis here #1230 (comment))