win,test: try again if file unlink fails

Now that parallel tests are enabled, the test runner spits out a ton of 'access denied' errors while running the tests. These happen because a virus scanner or the indexing service temporarily open the file after it has been updated, and the test runner tries to unlink() them at the same time.

This patch resolves this issue by attempting to unlink the file until it succeeds.

R=@bnoordhuis