Deduplicate logic of tests in `bigcurve_test.nr`

[TomAFrench]

There's an absolute ton of copy-pasted code in bigcurve_test.nr such that I find this file incredibly counter-productive.

Each test is repeated N times, once per curve instantiation, so we should have a generic implementation of the test which can then be called into by thin wrappers which inject the curve config.

A number of tests just check the curve config for consistency so we should then move those tests into the relevant curve configuration files. e.g. test_vesta_num_scalar_slices_in_scalar_field is just testing that VESTA_SCALAR_SLICES is set correctly so should be moved closer to where that global is defined.