runtime: disable GC around TestGoroutineParallelism

TestGoroutineParallelism can deadlock if the GC runs during the
test. Currently it tries to prevent this by forcing a GC before the
test, but this is best effort and fails completely if GOGC is very low
for testing.

This change replaces this best-effort fix with simply setting GOGC to
off for the duration of the test.

Change-Id: I8229310833f241b149ebcd32845870c1cb14e9f8
Reviewed-on: https://go-review.googlesource.com/10454Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/proc_test.go

@@ -7,6 +7,7 @@ package runtime_test
 import (
 	"math"
 	"runtime"
+	"runtime/debug"
 	"sync"
 	"sync/atomic"
 	"syscall"
@@ -104,8 +105,8 @@ func TestGoroutineParallelism(t *testing.T) {
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(P))
 	// If runtime triggers a forced GC during this test then it will deadlock,
 	// since the goroutines can't be stopped/preempted.
-	// So give this test as much time as possible.
-	runtime.GC()
+	// Disable GC for this test (see issue #10958).
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
 	for try := 0; try < N; try++ {
 		done := make(chan bool)
 		x := uint32(0)