testing: panic in AllocsPerRun if parallel tests are running

testing: panic in AllocsPerRun if parallel tests are running

If other tests are running, AllocsPerRun's result will be inherently flaky.
Saw this with CL 630136 and #70327.

Proposed in #70464.

Fixes #70464.

Change-Id: I190afdf26bc31299f6e5e8665b4fb420ffd554ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/630137
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>