Avoid creation of Pixels files in chgrp and delete tests

There are several ways of creating images within the python integration tests:
 * `new_image` - just create an Image object,
 * `createTestImage` - create a Pixels file on disk and link it to an Image,
 * `importSingleImage` - import a fake single series image,
 * `importMIF` - import a fake multi-image file,
 * `import_image` - import a file, fake or real.

This PR replaces `createTestImage` to avoid having Pixels files created where they are not necessary for the tests. No more or fewer tests fail than did before. The failure reasons are the same as before.

This PR could be merged as is but it might also be worth addressing the use of `createTestImage` throughout the tests, as well as tidying up other Image creation steps here and there. 

Also, would it be worth looking at replacing `createTestImage` with an fs-agnostic version? Maybe creating a fake image file with specified real Pixels and then importing that?