runtime: don't let the tests leave core files behind
Also add a check that we didn't leave any core files behind.
Change-Id: I30444ef43ad1a8cc1cacd3b75280f2128e104939
Reviewed-on: https://go-review.googlesource.com/c/go/+/525175 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org>