runtime: use small struct TestSynctest to ensure cleanups run
Finalizers and cleanup funcs weren't running on the windows-arm64
builder. Put finalizers/cleanups on a small struct containing a pointer
rather than an *int, which fixes the problem.
Also uncomment a synctest.Wait that was accidentally commented out.
Fixes #73977
Change-Id: Ia6f18d74d6fccf2c5a9222317977c7458d67f158
Reviewed-on: https://go-review.googlesource.com/c/go/+/679696
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>