ASTC decoding part 0: Test infrastructure improvements

[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 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

This is the first step a series of PRs that will add ASTC decoding support. This fixes a couple of small test/build related issues and improves test related convenience methods.

Fixes:

• Fix a race condition with file name collisions (caused DDS test failures around 75% of the time when run locally)
• Correctly dispose images in tests
• Suppress strong naming warning for interactive tests
• Fix build paths for non-Windows systems

Improvements:

• Standardize the existing KTX1 test to follow conventions and improve assertions
• Support test data subdirectories (WithFileAttribute walks subdirs)
• Add a new convenience method for comparing cubemap face (CompareFacesToReferenceOutput)

[Erik-White]

This is not new, just renamed from KtxDecoderTests.cs, but the test has been updated and improved