Remove helper functions for user input checks in tests, when they are not needed

[zinaStef]

Remove process_prior, process_simulator and check_user_input in tests whenever they are not needed, i.e. when we are using standard priors and simulators like MVN and the linear_gaussian simulator.

Related to PR #1083

Note: there might be a couple of occasions, where we actually need to use these helpers in the tests, namely when we are explicitly testing them e.g., when we are using improper priors or simulators on purpose, probably in user_input_checks_tests.py