Disable mount tests in Darwin to workaround panic: timedout bug

These tests reliably cause a "panic: timed out" condition due to
hitherto unknown reasons. See issue #3200 for context.

dlorenc mentioned that this test never worked, so when I re-enabled it
recently for all platforms in af61bf79,
it introduced a persistent test failure for darwin.

I'll leave issue #3200 open until we are able to re-enable this test.

test/integration/mount_test.go

@@ -34,6 +34,9 @@ import (
 
 func testMounting(t *testing.T) {
 	t.Parallel()
+	if runtime.GOOS == "darwin" {
+		t.Skip("mount tests disabled in darwin due to timeout (issue#3200)")
+	}
 	if strings.Contains(*args, "--vm-driver=none") {
 		t.Skip("skipping test for none driver as it does not need mount")
 	}