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/77130Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -5093,6 +5093,9 @@ func TestGoTestMinusN(t *testing.T) {
 }
 
 func TestGoTestJSON(t *testing.T) {
+	if runtime.NumCPU() == 1 {
+		t.Skip("skipping on uniprocessor")
+	}
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.parallel()