Spectrum Viewer: Improve Testing of Stack and Unit Mode Change

[JackEAllen]

Desired Behaviour

A bug was recently found in the spectrum viewer where tof_mode was not set correctly on initialisation when opening with a new dataset which included a spectra file (SEE PR: #2212) Tests did not fail meaning this bug was only found by chance during usage.

We should add some more tests surrounding swapping sample stacks and unit modes where the sample being switched too and from may or may not have a spectra log file loaded.

Example tests we should add:

• [x] Switching stack with and without data behave correctly
• [x] Opening spectrum viewer first time without spectra data
• [x] Opening spectrum viewer with spectra data
• [x] Switching from a stack with a spectra file to a stack without a spectra file
• [x] Switching from a stack without a spectra file to a stack with a spectra file
• [x] Switching form a stack without a spectra file to another stack without a spectra file
• [x] Switching from a stack with a spectra file to another stack with a spectra file for each ToF mode, ensuring mode stays the same when switching stacks.
• [ ] ToF mode should never be None. if None, an error should be returned

Current Behaviour

There are no tests to check swapping between samples and/or unit modes.

[samtygier-stfc]

Once #2239 is merged I am happy that we have enough type checking to be confident that the TOF mode should never get set to None.