Add UASTC and RAW file extensions for test data

[Erik-White]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Adds UASTC and RAW so they can be used in test data for UASTC support in ImageSharp.Textures:
SixLabors/ImageSharp.Textures#58

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 18%. Comparing base (7596697) to head (c5beec2).
⚠️ Report is 38 commits behind head on main.

Additional details and impacted files

@@        Coverage Diff         @@
##           main   #65   +/-   ##
==================================
- Coverage    19%   18%   -1%     
==================================
  Files         4     4           
  Lines       346   332   -14     
  Branches     85    81    -4     
==================================
- Hits         68    62    -6     
+ Misses      278   270    -8     

Flag	Coverage Δ
unittests	18% <ø> (-1%)	⬇️

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