runtime/trace: fix TestTraceSymbolize when GOMAXPROCS=1

CL 92916 added the GOMAXPROCS test in TestTraceSymbolize.

This test only succeeds when the value of GOMAXPROCS changes.

Since the test calls runtime.GOMAXPROCS(1), it will fails
on machines where GOMAXPROCS=1.

This change fixes the test by calling runtime.GOMAXPROCS(oldGoMaxProcs+1).

Fixes #23816.

Change-Id: I1183dbbd7db6077cbd7fa0754032ff32793b2195
Reviewed-on: https://go-review.googlesource.com/93735
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/trace/trace_stack_test.go

@@ -128,7 +128,8 @@ func TestTraceSymbolize(t *testing.T) {
 	wp.Write(data[:])
 	<-pipeReadDone
 
-	oldGoMaxProcs := runtime.GOMAXPROCS(1)
+	oldGoMaxProcs := runtime.GOMAXPROCS(0)
+	runtime.GOMAXPROCS(oldGoMaxProcs + 1)
 
 	Stop()