Add test assets for CachedBitmap tests

[reflectronic]

These files were taken from the equivalent tests in libgdiplus.

[safern]

@reflectronic where did these assets come from? Trying to figure out if there is any license implication here.

[reflectronic]

The original file is this one: https://github.com/mono/libgdiplus/blob/master/tests/test.png
The edited versions can be found in my PR: https://github.com/mono/libgdiplus/pull/654/files#diff-45dfcbc04d748ef89d3aedd77b182a02

[safern]

Great. Thanks 😄