test(test): fix inconsistent timezone in date tests

[dabbott]

I noticed a few date-related tests weren't passing for me locally:

 FAIL  test/unsafe.test.ts
  ● change Date instance

    expect(received).toBe(expected) // Object.is equality

    Expected: 946684800000
    Received: 946627200000

      577 |   expect(data.foo).toBe(state.foo);
      578 |   expect(data.date).toBe(state.date);
    > 579 |   expect(state.date.getTime()).toBe(946684800000);
          |                                ^
      580 | });
      581 |

      at Object.<anonymous> (test/unsafe.test.ts:579:32)

I believe this is because setFullYear applies the system's local time zone automatically. Using setMilliseconds should avoid this issue. With this change, all tests pass locally and presumably shouldn't impact the github actions test runner.

I love the library and I'm excited to start poking around the code more!