runtime: log stderr in TestLockRankGenerated on failure

runtime: log stderr in TestLockRankGenerated on failure

For #62334.

Change-Id: I6d93dacc914590ef9c5a034dd4633466ec0bde8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/524056
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>