cmd/go: skip TestGoTestJSON on uniprocessor machines
cmd/go: skip TestGoTestJSON on uniprocessor machines

CL 76873 added TestGoTestJSON. However, this test
is only succeeding on SMP machines.

This change skips TestGoTestJSON on uniprocessor machines.

Fixes #22665.

Change-Id: I3989d3331fb71193a25a3f0bbb84ff3e1b730890
Reviewed-on: https://go-review.googlesource.com/77130
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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