Occasionally this test fails in CI with the message:
```
assertion failed: (current_time / 1000000 - last_time / 1000000 == 1)
```

The way this calculation is done at the moment, a difference of 1001ms
between `current_time` and `last_time` can result in failure, if the
times are close to a multiple to 1000ms.

Change it to only truncate the result after doing the subtraction, and
add a 500ms tolerance to account for scheduling delays in the test. (For
example, the `test_func()` could be called, then descheduled before it
gets to call `g_get_monotonic_time()`.

Additionally, change the test to use `g_assert_cmpint()` so that future
failures provide more useful debug information.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>