runtime: relax TestMemoryLimitNoGCPercent a bit

It seems to be pretty flaky. I've seen:

retained=289438024
   limit=268435456
   bound=285212672

Which is ~4MB over the bound.

Not sure why this tends to be darwin-specific, but we'll fix
just darwin for now.
(It isn't quite darwin-only, as it appeared in #66893.
But it is certainly worse on darwin.)

Fixes #73136
Update #66893

Change-Id: If609e909bc6c65c2663dd46b7a9bad4fd291c3da
Reviewed-on: https://go-review.googlesource.com/c/go/+/689315
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/testdata/testprog/gc.go

@@ -395,6 +395,9 @@ func gcMemoryLimit(gcPercent int) {
 		// somewhat heavily here) this bound is kept loose. In practice the Go runtime
 		// should do considerably better than this bound.
 		bound := int64(myLimit + 16<<20)
+		if runtime.GOOS == "darwin" {
+			bound += 16 << 20 // Be more lax on Darwin, see issue 73136.
+		}
 		start := time.Now()
 		for time.Since(start) < 200*time.Millisecond {
 			metrics.Read(m[:])