Fix timezone-dependent symlink test in TestFileProcessorHandler

[shreyamalviya]

On running the unit tests, test_symlink_latest_log_directory was failing for me because of a timezone issue (I'm in BST/UTC+1).

The tests compute the expected log-directory name from timezone.utcnow() but then travel to that date with a naive date string: time_machine.travel("2026-06-24", ...). time_machine interprets a naive datetime as local time, so under a positive UTC offset, the travel target lands on the previous day in UTC (2026-06-24 00:00 BST == 2026-06-23 23:00 UTC). The handler then builds its directory from utcnow() (2026-06-23) while the assertion expects 2026-06-24, producing:

AssertionError: assert '2026-06-23' == '2026-06-24'

This is a latent test bug that surfaces for any contributor in a positive-offset timezone.

The fix makes the time_machine.travel targets explicit UTC instants (f"{date} 00:00:00+00:00") so the day no longer shifts with the host timezone. Behaviour under test is unchanged; only the test's time control is made timezone-independent.

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