Add unit tests for AssetManager

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	❌
Tests added?	✔️
Breaks BC?	❌
Fixed issues

What does this PR do?

Adds AssetManagerTest - unit tests for AssetManager.

Coverage

69 tests. Line coverage: 149/149 (100%). MSI: 94% (265/281).

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.88%. Comparing base (279108f) to head (e146716).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20789      +/-   ##
============================================
+ Coverage     78.68%   78.88%   +0.20%     
  Complexity    11566    11566              
============================================
  Files           376      376              
  Lines         30748    30748              
============================================
+ Hits          24194    24256      +62     
+ Misses         6554     6492      -62     

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