Test suite tooling

[VeckoTheGecko]

Currently the test suite is quite disconnected, where each test module is self reliant to generate its own Fields/FieldSets and fixtures for testing.

This PR introduces tooling via the tests.utils and tests.common_kernels modules for the sharing of code (utility functions, kernels, and test data generation) between tests. This has multiple benefits:

• Better organisation/code overview, enabling a standard collection of FieldSets to be used for testing
  • Reduces the size of the test suite, as calls toField.from_data() aren't necessary in most cases. Also helping readability
  • Makes it easier to write tests, as selecting test data is less of a concern
• Kernels used commonly in tests can be imported instead of rewritten.

Other changes:

• Move fieldset generation and provide quick documentation
• Remove redundant fieldset
• Cleans test functions' arguments (such that only pytest params and fixtures are there. Test constants are moved to function body)
• Update filepath handling for test data reducing code duplication

Further refactoring of tests - including the use of common fieldsets, and creating global fixtures in a conftest.py - can be done in future in a piecemeal approach as needed.

Related to #1684