Fix SquashFS test fixtures when mksquashfs lacks xattr support

[Olof-Lagerkvist]

When mksquashfs is installed but was built without xattr support, SquashFixtures still requests -xattrs-add for its fourth image. The resulting error aborts the shared setup, so unrelated SquashFS reader tests fail and repeatedly rebuild the other three images. This matches the reported FreeBSD failure.

The setup now caches generated image paths individually. The three -no-xattrs fixtures are still built fresh. Only when the optional xattr image returns exit code 1 with the exact unsupported-xattrs diagnostic does it use the existing embedded xattr fixture, discarding any partial output. The xattr reader test continues to run; other tool errors remain failures.

Validation performed on Linux:

• Reproduced the original failure with native squashfs-tools 4.7.5 built without XATTR_SUPPORT.
• All 19 SquashFS reader/builder tests passed in each configuration: xattrs enabled, xattrs disabled, and mksquashfs absent (57 successful test executions).
• Invocation tracing confirmed one shared setup per run, with the three ordinary images generated in both native-tool configurations.
• Negative controls confirmed that an unrelated xattr-build error and the unsupported-xattrs diagnostic during an ordinary-image build still fail.
• LibraryTests Release builds passed for net46, net48 and net10.0 with zero warnings or errors.

FreeBSD execution has not been performed here. No library code or embedded images are changed.