minor fixes in tests required to test xcoll in xsuite workflows

[szymonlopaciuk]

Description

Minor fixes, mostly in tests, in preparation for setting up automated testing for xcoll.

• changed some tolerances in test_everest.py
• changed an assertion in test_lossmap.py, where in the cases when ignore_crystals=False there was more elements.
• made some adaptations to tests to make them pass on GPU (including excluding two in test_collimator_elements.py
• changed abs to fabs where appropriate
• fixed the usage of @retry() (currently the brackets are necessary even if no parameters are provided, otherwise strange things happen...)
• specified extras installation for tests as for the other xsuite packages. When pip install xcoll[tests] is run, pytest and ruamel.yaml are also installed.
• made tests independent from the current working directory.

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts