Refactor eagroup_context_test for readability and speed

[robertapplin]

Description of work

This PR does a refactor of the elemental_analysis_eagroup_context_test to improve its readability and its speed. There are several changes made to this test:

• The detector workspaces are only created once inside the setUpClass method (previously they were created 4 times).
• The Context and MuonLoadData objects are cleared in the tearDown method to avoid code duplication
• The Given-When-Then structure is used to improve the readability of the test https://pythontest.com/strategy/given-when-then-2/
• I avoid storing the workspaces in the ADS immediately until they are required. This makes it more explicitely obvious that the workspaces are required in the ADS (because the context is unfortunately currently dependent on the ADS)

To test:

Code review and check the test is passing on all PR operating systems jobs

This does not require release notes because it is a refactor of a unit test

Reviewer

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[Despiix]

Had a look at the changes and it looks good! Good to go if it passes all tests.

[sf1919]

I've looked through the code and the changes make sense to me. The tests pass so I will merge it in.