Export ktxTexture2_DecodeAstc in JS bindings

[MarkCallow]

Add ktxTexture2_DecodeAstc to libktx_read, a previous omission. Needed too so the function can be included in the read-only JS bindings.

Add tests of ASTC encode and decode functionality to texturetests. The change includes needing to pass the path to ktxdiff to texturetests as the new tests use it to compare the decoded result with the original image.