runtime: skip TestSegv failures with "morestack on g0" on darwin/amd64

This failure mode has been present since at least 2020-06-08. We have
enough information to diagnose it, and further failures don't seem to
be adding any new information at this point: they can only add noise,
both on the Go project's builders and in users' own modules (for
example, when run as part of 'go test all').

For #39457

Change-Id: I2379631da0c8af69598fa61c0cc5ac0ea6ba8267
Reviewed-on: https://go-review.googlesource.com/c/go/+/382395
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: Bryan C. Mills

src/runtime/crash_cgo_test.go

@@ -596,6 +596,9 @@ func TestSegv(t *testing.T) {
 			t.Log(got)
 			want := "SIGSEGV"
 			if !strings.Contains(got, want) {
+				if runtime.GOOS == "darwin" && runtime.GOARCH == "amd64" && strings.Contains(got, "fatal: morestack on g0") {
+					testenv.SkipFlaky(t, 39457)
+				}
 				t.Errorf("did not see %q in output", want)
 			}