Tests: Add imbuf tests for the DDS image format

Add coverage for loading in BC1, BC3, BC4, BC5, and BC7 variants.
Note: BC2 (DXT3) is still missing.

Pull Request: https://projects.blender.org/blender/blender/pulls/157938

tests/python/bl_imbuf_load.py

@@ -168,6 +168,9 @@ class ImBufLoadTest(ImBufTest):
     def test_load_iris(self):
         self.check("*.rgb")
 
+    def test_load_dds(self):
+        self.check("*.dds")
+
 
 class ImBufBrokenTest(AbstractImBufTest):
     @classmethod