update default mock_NWBFile start time

[stephprince]

Motivation

Fix #2013. There was an error when creating a mock_NWBFile in specific time zones, where converting the default start time to local time would result in a time before the Unix epoch.

The default could also be updated to any later time, I left as 1970 for now in case that is used to indicate this is mock data.

I don't think these changes require a CHANGELOG update but can add if needed.

How to test the behavior?

from pynwb import NWBHDF5IO
from pynwb.testing.mock.file import mock_NWBFile

nwb = mock_NWBFile()

with NWBHDF5IO('test.nwb', mode='w') as io:
    io.write(nwbfile)

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.73%. Comparing base (739ee54) to head (a1c4112).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2018   +/-   ##
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  Coverage   91.73%   91.73%           
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  Files          27       27           
  Lines        2722     2722           
  Branches      710      710           
=======================================
  Hits         2497     2497           
  Misses        149      149           
  Partials       76       76           

Flag	Coverage Δ
integration	72.96% <ø> (ø)
unit	82.29% <ø> (ø)

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[h-mayorquin]

Hey guys, slightly related but now that python 3.9 is minimal have you considered using ZoneInfo from the standard library instead of dateutil?

https://docs.python.org/3/library/zoneinfo.html

Eventually you can eliminate another dependency.