Improve tests to allow parallel running

[GCHQDeveloper314]

Describe the new feature you'd like Unit Tests should be able to be run in parallel. For this to work, some tests need to be improved or converted to integration tests. This is because they are either inconsistent and depend on prior tests or use the filesystem or similar dependencies which should really classify them as integration tests instead.

Why do you want this feature? This will have the potentially to reduce the time taken to run unit tests significantly.

Describe alternatives you've considered Using surefire plugin options of <parallel>classes</parallel> and <forkCount>2</forkCount> or <threadCount>2</threadCount> causes test failures in some modules. See the surefire docs for more info on the difference between these options. Work is needed to fix the root causes of these errors or to part failing classes with an annotation to ensure they are not run in parallel when these options are used.

Additional context This issue is likely blocked until #2702 has been resolved. It might be beneficial to do it alongside #1655 .