sync: set GOMAXPROCS to 1 in TestPoolGC

This test expects to be able to drain a Pool using only Get. This isn't
actually possible in the general case, since a pooled value could get
stuck in some P's private slot. However, if GOMAXPROCS=1, there's only 1
P we could be running on, so getting stuck becomes impossible.

This test isn't checking any concurrent properties of Pool, so this is
fine. Just set GOMAXPROCS=1 for this one particular test.

Fixes #73728.

Change-Id: I9053e28118060650f2cd7d0d58f5a86d630b36f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/673375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/sync/pool_test.go

@@ -102,6 +102,18 @@ func TestPoolRelease(t *testing.T) {
 }
 
 func testPool(t *testing.T, drain bool) {
+	if drain {
+		// Run with GOMAXPROCS=1 if drain is set. The code below implicitly
+		// assumes it can remove all the pool-cached values with cleanups
+		// with Get, but this isn't necessarily true if a value gets stuck
+		// in the private slot for some P. This is especially likely when
+		// running with mayMoreStackPreempt. We can make this exact, however,
+		// by setting GOMAXPROCS to 1, so there's only 1 P. This is fine for
+		// this test, since we're not trying to check any concurrent properties
+		// of Pool anyway.
+		defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
+	}
+
 	var p Pool
 	const N = 100
 	for try := 0; try < 3; try++ {