Expose starting time in mock_ElectricalSeries

[h-mayorquin]

Motivation

The starting_time is exposed in all the mock methods but for some reason not in mock_ElectricalSeries. I needed this recently in a test in spike interface so I am pushing it here in case you want to add it.

I also realized that the mock_OnePhotonSeries function has offset exposed but not propagated whereas mock_TwoPhotonSeries had neither of them. I did this as well.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (17b5621) 91.99% compared to head (6598cdc) 83.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1805      +/-   ##
==========================================
- Coverage   91.99%   83.68%   -8.32%     
==========================================
  Files          27       27              
  Lines        2623     2623              
  Branches      685      685              
==========================================
- Hits         2413     2195     -218     
- Misses        138      344     +206     
- Partials       72       84      +12     

Flag	Coverage Δ
integration	?
unit	83.68% <ø> (ø)

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