Fixed test failing for wagtail 5.2

[JakubMastalerz]

Closes https://github.com/torchbox/wagtail-grapple/issues/377

This fixes test_src_set_invalid_format for Wagtail versions above 5.2 by:

• Adding a new test case for those versions, with updated error message.
• Adding conditional skipping based on active Wagtail version.