[Python] Test timestamp from int without pandas dependency

[HyukjinKwon]

Describe the enhancement requested

The test test_sequence_timestamp_from_int_with_unit() was marked with
@pytest.mark.pandas, meaning it was skipped when pandas was not installed.
This left the non-pandas code path untested.

The TODO was introduced in commit 286bf7c when making pandas
an optional dependency:

arrow/python/pyarrow/tests/test_convert_builtin.py

Lines 731 to 732 in 286bf7c

	# TODO(wesm): This test might be rewritten to assert the actual behavior
	# when pandas is not installed

Component(s)

Python

[AlenkaF]

Issue resolved by pull request 48655
#48655