[test] Rename existing external_texture tests to external_image_copy

These existing tests cover `Queue::copy_external_image_to_texture()`
which, despite the similar name, is unrelated to the external texture
binding resource.

The following patch is going to add some tests for the latter, so we
are taking this opportunity to rename the former to help avoid
confusion.

tests/tests/wgpu-gpu/main.rs

@@ -28,7 +28,7 @@ mod dispatch_workgroups_indirect;
 mod draw_indirect;
 mod dual_source_blending;
 mod encoder;
-mod external_texture;
+mod external_image_copy;
 mod float32_filterable;
 mod image_atomics;
 mod instance;