Skip tests when dials_data not available
Skip tests when dials_data not available

This covers both cases of the python package missing and the pytest
plugin being disabled. It will not make pytest accept a '--regression'
parameter. The default approach should remain to install dials_data
whenever possible.

Resolves #1256

dials/dials,dials/dials,dials/dials,dials/dials,dials/dials