Add unit tests for merging rows into the reflectometry GUI

[gemmaguest]

This PR adds additional unit tests to do with merging rows into the ISIS Reflectometry GUI. In particular, it tests the mergeRowIntoGroup function, which updates the ReductionJobs model with the merged row (if a merge is applicable).

There is also some minor tidying to simplify the function interface and remove spurious warnings from existing tests.

To test:

• Code review and ensure unit tests pass.
• Ensure transferring rows still works in the ISIS Reflectometry interface: follow the same tests as on PR Add tests for parsing search results to reflectometry GUI #26517

Refs #26512

This does not require release notes because it concerns testing 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 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.

[dtasev]

Tests look good, pass locally and on build servers. Some merge conflicts have arisen though

[dtasev]

I've rebuilt the failed Windows build 31771

[dtasev]

Looks good!