FILE_FLAG_DELETE_ON_CLOSE had the wrong semantics, rendering the lock
file unusable after it had been closed once.
Delete the lock file in the uninstaller as a simple alternative (given that
the lock file is not in a temporary location on Windows).
For a test I returned to the older form of
test_backgroundtask_update_sync_manager which initially exposed the issue:
It expects the background task to be able to detect the xpcshell instance
after running resetLock, which failed before this fix.
I also extended the original updateSyncManager test to run the second
copy twice, which also catches the issue.
Differential Revision: https://phabricator.services.mozilla.com/D109565
6b6c4da4b5