Nexgen 231 thoroughly test nexgen created files against gda

[rtuck99]

Does part of DiamondLightSource/nexgen#231

Link to dodal PR (if required): N/A (remember to update setup.cfg with the dodal commit tag if you need it for tests to pass!)

To test:

Unit tests should pass

The expected/exceptional vaues have NOT been verified; the values have been entered based on what the code currently does, on the expectation that it works.

Also perhaps we should split into two tests, one against GDA old data, and one against nexus standard conformance.

Another Q is, whether this test or something like it ought to be in nexgen rather than here - I am open to suggestions

[dperl-dls]

I would like to merge this to add on to it but leave the issue open until we're sure about the contents of the exceptions etc