internal/trace: skip async preempt off tests on low end systems

The OpenBSD armv7 port does not support SMP - on this platform the
trace tests take ~300 seconds to run when async preempt is disabled,
which then times out on the builder. Skip these tests when run in
short mode on a single CPU system.

Change-Id: I9a697d5ba2b20652f76dcc97bd178a4ee8f1a2a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/698555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/internal/trace/trace_test.go

@@ -694,6 +694,9 @@ func testTraceProg(t *testing.T, progName string, extra func(t *testing.T, trace
 		runTest(t, false, "")
 	})
 	t.Run("AsyncPreemptOff", func(t *testing.T) {
+		if testing.Short() && runtime.NumCPU() < 2 {
+			t.Skip("skipping trace async preempt off tests in short mode")
+		}
 		runTest(t, false, "asyncpreemptoff=1")
 	})
 	t.Run("Stress", func(t *testing.T) {