tape: keep custom expect messages

[milahu]

fix #106 by preserving the custom expect message

sample

-  t.equal(1+1, 2, '1+1 should be 2');
+  expect(1+1, '1+1 should be 2').toBe(2);

[skovhus]

I'm not convinced we should support this use case as isn't supported natively by Jest and requires the custom jest-expect-message dependency (18K weekly downloads compared to Jest with 14M weekly downloads).