runtime: skip TestGdbBacktrace on gdb bug

Very rarely, GDB will successfully run the whole test and the inferior
will exit successfully, and then GDB itself hangs and never exits.
Detect this and skip the test as flaky. We could just continue the
test since all of the output we need is there, but by skipping it
we're less likely to notice serious regressions in this test.

Fixes #37405.

Change-Id: I016cbb06f48673f064733da3e3f1ddcbefd58159
Reviewed-on: https://go-review.googlesource.com/c/go/+/411117
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/runtime-gdb_test.go

@@ -435,6 +435,11 @@ func TestGdbBacktrace(t *testing.T) {
 			// GDB bug: https://sourceware.org/bugzilla/show_bug.cgi?id=9086
 			testenv.SkipFlaky(t, 50838)
 		}
+		if bytes.Contains(got, []byte(" exited normally]\n")) {
+			// GDB bug: Sometimes the inferior exits fine,
+			// but then GDB hangs.
+			testenv.SkipFlaky(t, 37405)
+		}
 		t.Fatalf("gdb exited with error: %v", err)
 	}