runtime: mark TestGdbCoreCrashThreadBacktrace as flaky

runtime: mark TestGdbCoreCrashThreadBacktrace as flaky

This test exercises the SIGQUIT crash loop and managed to trigger the
race from #65138 at least once.

For #65138.
Fixes #64752.

Change-Id: I11091510aa7ae4f58b1d748e53df2e3e3dbfb323
Reviewed-on: https://go-review.googlesource.com/c/go/+/556356
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>