Tests: Use image-neutral wording in camera CLI logs

The camera CLI test now runs for both PNG and PPM builds, so the single/movie
comparison messages should say "image" rather than "PNG".

tests/test_camera_cli.py

@@ -207,7 +207,7 @@ with tempfile.TemporaryDirectory(prefix='gr-camera-cli-') as directory:
             '--frames-prefix', backend, '--duration', 0, '--fps', 1,
             '--lens-map-output', movie_map)
         movie = image_payload(tmp / f'{backend}_000000.{ext}')
-        assert single == movie, f'{backend}: single/movie PNG mismatch'
+        assert single == movie, f'{backend}: single/movie image mismatch'
         a, ta = map_vertices(single_map)
         b, tb = map_vertices(movie_map)
         assert len(a) == len(b) and ta == tb
@@ -286,4 +286,4 @@ with tempfile.TemporaryDirectory(prefix='gr-camera-cli-') as directory:
                 assert parallel_map.read_bytes() == comparison_map.read_bytes(), \
                     f'movie lens map changed with {threads} threads'
             print('schwarzschild: movie lens map identical with 1, 4 and 16 threads', flush=True)
-        print(f'{backend}: CLI checks passed; single/movie PNG identical, map max error {max_error:.3g}', flush=True)
+        print(f'{backend}: CLI checks passed; single/movie image identical, map max error {max_error:.3g}', flush=True)