runtime: don't serialize all builds in test

runtime: don't serialize all builds in test

Permit a test whose program is already built to run immediately,
rather than waiting for another test to complete its build.

For #44422

Change-Id: I2d1b35d055ee4c4251f4caef3b52dccc82b71a1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/364654
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>