Indirect Unit tests for fitting models

[robertapplin]

Description of work.
This PR introduces unit tests for the various fitting models including ConvFitModel, JumpFitModel, IqtFitModel and MSDFitModel.

To test:
Make sure the tests pass.

No release notes - these are behind the scenes tests

Fixes #24649

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.

[TTitcombe]

Code changes are mostly new tests.
Test coverage seems good.
No need for release notes as internal tests.