Unify `DiracOperator` tests

[chillenzer]

The structure of the tests concerning the DiracOperator is mostly identical in all tests:

• Create a (potentially single-contribution) superposition of eigenfunctions.
• Apply DiracOperator.
• Check that each contributing eigenfunction received the correct factor.

At some point, we should unify all tests from test_{real,spectral}_basis.py into a single test the parametrisation of which takes care of testing all combinations of superpositions, input/output basis, spectra, etc.

[chillenzer]

Also, we want to remove conftest.py with this. (See here.