HTML import test for image inlined in 'src' attribute (related: fdo#50763)

Fixed in 72703173066a2db5c977d422ace59d60b998bbfc

Unfortunately the test does not really ensure it works now :-/
but still probably better than nothing and can be fixed potentially.

Change-Id: I563b46c62d256c58a63f36443a5431603169c3f6