Timeline::GetClip: Add anti-slicing checks to unit tests

[ferdnyc]

Defense against any recurrence of #595 breakage.

[codecov]

Codecov Report

Merging #596 (5e2c2bc) into develop (e6eef0d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #596      +/-   ##
===========================================
+ Coverage    52.54%   52.56%   +0.02%     
===========================================
  Files          129      129              
  Lines        10707    10710       +3     
===========================================
+ Hits          5626     5630       +4     
+ Misses        5081     5080       -1     

Impacted Files	Coverage Δ
tests/Timeline_Tests.cpp	100.00% <100.00%> (ø)
src/Clip.h	83.33% <0.00%> (+8.33%)	⬆️

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