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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go