Build Mantid and unit tests

[AndreiSavici]

Description of work.

To test:
Make sure that the builds pass. If you have Ubuntu 20.04, try to build on that. For things to work, set QT_API to pyqt5 (export QT_API="pyqt5")

Re #28650

This does not require release notes because is only partial way to transition to workbench only

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Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there is GUI work does it follow the GUI standards?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @mantidproject/gatekeepers will take care of it.

[martyngigg]

Confirmed all tests now build and the protection for a variable that might be None is sensible.