runtime: reject any goroutine leak test failure that failed to execute

runtime: reject any goroutine leak test failure that failed to execute

This is far more general than the regexp, which was necessary only
because runTestProg doesn't return the error. This change makes
runTestProg a wrapper function around a function that *does* return the
error.

For #76526.

Change-Id: Ib3daa75eb0fe314a28a7a368474943ba470d0d4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/726525
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>