[release-branch.go1.17] runtime: mark TestGcSys as flaky

I don't know what this test is doing, but it very frequently flakes
for me while testing mundane compiler CLs. According to the issue log,
it's been flaky for ~3 years.

Updates #37331.
Fixes #52826.

Change-Id: I81c43ad646ee12d4c6561290a54e4bf637695bc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/336349
Trust: Matthew Dempsky <[email protected]>
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
(cherry picked from commit d8ceb13)
Reviewed-on: https://go-review.googlesource.com/c/go/+/406974
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

src/runtime/gc_test.go

@@ -21,6 +21,7 @@ import (
 )
 
 func TestGcSys(t *testing.T) {
+	t.Skip("skipping known-flaky test; golang.org/issue/37331")
 	if os.Getenv("GOGC") == "off" {
 		t.Skip("skipping test; GOGC=off in environment")
 	}