Define a testhelpers Python package that sits on C++ one.

Refs #7057
Define a macro to check that Python unit tests will actaully run.

If they do not end with uniitest.main() then they will not be executed.
It is now a cmake error if they do not. Refs #7057
Use new macro and fix tests that were not being executed. Refs #7057
Add CreateEventWorkspace helpers to WorkspaceCreationHelper mod.

Refs #7057
Use workspace helpers instead of files for peaks & event workspaces.

Refs #7057
Export add methods on ADS to Python. Refs #7057
Replace more file loading with in-memory workspaces. Refs #7057
Include MDEventsTestHelpers inside Python testhelpers package.

Refs #7057
Export DateAndTime constructors to Python. Refs #7057
Replace file usage in RunTest.py

It uses the new DateAndTime constructors to simulate the load.
Refs #7057
Remove file from TimeSeriesProperty test. Refs #7057
Remove more files from Python tests. Refs #7057
Replace files in simpleapi tests (where possible). Refs #7057

The SimpleAPILoad has to load files to check Load is doing its job
properly so they cannot be removed.
Remove use of file in CreateWorkspaceTest. Refs #7057

The remaining algorithm tests require file loading so will not be touched.
Merge branch 'feature/7057_py_unittests_rm_files' into develop