Backport CL 421442 from upstream.

Original description:

Arrange for tests that call setMimeInit to fully restore the old values,
by clearing the sync.Once that controls initialization.

Once we've done that, call initMime in initMimeUnixTest because
otherwise the test types loaded there will be cleared by the call to
initMime that previously was not being done.

For golang/go#51648

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/483117