Test png icon if svg fails to load

[bynect]

Made valid.png 16x16 to be consistent with valid.svg and reworked notification_load_icon_with_scaling to use png if svg fails to load (this is due the missing librsvg which is a runtime dependency)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 65.68%. Comparing base (07b68f0) to head (f107bba).
Report is 3 commits behind head on master.

Files	Patch %	Lines
test/notification.c	61.53%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1329      +/-   ##
==========================================
- Coverage   65.71%   65.68%   -0.03%     
==========================================
  Files          50       50              
  Lines        8041     8047       +6     
  Branches      925      928       +3     
==========================================
+ Hits         5284     5286       +2     
- Misses       2757     2761       +4     

Flag	Coverage Δ
unittests	65.68% <61.53%> (-0.03%)	⬇️

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