Image tests are slow

[nbachiyski]

Currently the tests in tests/test_image.php are only a few, but they run for 3 seconds.

They're slow, because they run on actual images, but we don't actually test parsing the image file themselves. We are testing how the functions cope with the data given by the PHP image functions.

Solution: test only the logic in the function, not how well the PHP image functions work.

For this we will need to refactor the WordPress? image functions to allow to be testable.

This may be relevant to the core ticket ​#6821