[Python] test_assume_timezone does not assert nonexistent and ambiguous results

### Describe the bug, including details regarding any error messages, version, and platform.

`test_assume_timezone` in `python/pyarrow/tests/test_compute.py` performs four `.equals()` comparisons without asserting their boolean results. The comparisons for nonexistent times (`shift_forward` and `shift_backward`) and ambiguous times (earliest and latest) are therefore discarded, allowing incorrect kernel output on those branches to pass the test.

Expected: each computed result is asserted against the expected timestamp array.

Actual: the four comparisons execute but verify nothing.

### Component(s)

Python