[Python] test_assume_timezone does not assert nonexistent and ambiguous results

[anxkhn]

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