Fix avhrr_l1b_gaclac tests

[sfinkens]

Fix avhrr_l1b_gaclac tests by adapting imports to the latest Pygac release and creating a more realistic telemetry array. Claude helped me understanding the 10bit encoding.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.30%. Comparing base (97d6314) to head (0dbd7e5).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3398      +/-   ##
==========================================
- Coverage   96.32%   96.30%   -0.02%     
==========================================
  Files         466      466              
  Lines       59090    59096       +6     
==========================================
- Hits        56917    56914       -3     
- Misses       2173     2182       +9     

Flag	Coverage Δ
behaviourtests	3.59% <0.00%> (-0.01%)	⬇️
unittests	96.39% <100.00%> (-0.02%)	⬇️

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[djhoese]

I found an edge case apparently in one of the tests:

[gw0] linux -- Python 3.11.15 /home/runner/miniconda3/envs/test-environment/bin/python3.11
    def test_time_seconds():
        """Test conversion of timecode to datetime64."""
        current = datetime.now()
>       current = current.replace(microsecond=round(current.microsecond, -3))
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       ValueError: microsecond must be in 0..999999
current    = datetime.datetime(2026, 5, 20, 14, 2, 28, 999833)
satpy/tests/reader_tests/test_avhrr_l0_hrpt.py:193: ValueError

[pnuu]

LGTM

[djhoese]

Looks like we're now only failing unstable CI with tests from test_mviri_l1b_fiduceo_nc.py. Thanks @sfinkens!