cmd/go: deflake TestGoTestJSON further
cmd/go: deflake TestGoTestJSON further

Even after disabling on 1-CPU systems, builders are still flaking too often.
Unless there are at least 4 CPUs, don't require test interlacing at all.

Fixes #22665 (again).

Change-Id: Ief792c496c1ee70939532e6ca8bef012fe78178e
Reviewed-on: https://go-review.googlesource.com/77310
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

Cofyc/go,golang/go,Cofyc/go,codestation/go,golang/go,golang/go,josharian/go,Cofyc/go,josharian/go,josharian/go,josharian/go,codestation/go,CAFxX/go,codestation/go,nawawi/go,josharian/go,CAFxX/go,CAFxX/go,Cofyc/go,golang/go,golang/go,Cofyc/go,Cofyc/go,akutz/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,codestation/go,christopher-henderson/Go,CAFxX/go,golang/go,akutz/go,codestation/go,codestation/go,josharian/go,Cofyc/go,josharian/go,Cofyc/go,codestation/go,akutz/go,nawawi/go,cloudflare/go,codestation/go,josharian/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go