tests: Add a suite of basic UI tests

[andyholmes]

No description provided.

[codecov]

Codecov Report

Merging #6 (daa85f5) into master (dcf9fc8) will increase coverage by 3.39%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   36.82%   40.22%   +3.39%     
==========================================
  Files         242      242              
  Lines       16596    16583      -13     
  Branches     2803     2800       -3     
==========================================
+ Hits         6112     6670     +558     
+ Misses       8547     7844     -703     
- Partials     1937     2069     +132     

Impacted Files	Coverage Δ
src/libvalent/ui/valent-device-panel.h	0.00% <ø> (ø)
src/libvalent/ui/valent-plugin-group.h	0.00% <ø> (ø)
src/libvalent/ui/valent-plugin-row.c	85.71% <ø> (+85.71%)	⬆️
src/libvalent/ui/valent-plugin-row.h	0.00% <ø> (ø)
src/libvalent/ui/valent-device-panel.c	71.85% <33.33%> (+71.85%)	⬆️
src/libvalent/ui/valent-plugin-group.c	78.57% <100.00%> (+78.57%)	⬆️
src/libvalent/ui/valent-panel.h	0.00% <0.00%> (ø)
src/libvalent/ui/valent-window.h	0.00% <0.00%> (ø)
src/libvalent/ui/valent-menu-list.h	0.00% <0.00%> (ø)
src/libvalent/ui/valent-menu-stack.h	0.00% <0.00%> (ø)
... and 13 more

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