Script Generator: Squish test dynamic scripting

As a user, I would like squish tests to confirm the dynamic scripting behaviour introduced in https://github.com/ISISComputingGroup/IBEX/issues/6841.

### Acceptance Criteria

- [ ] Squish tests adequately cover behaviour and possible error states

### Extra Information
_Where required files/links are_
Expected behaviour is documented here: https://github.com/ISISComputingGroup/System_Tests_UI_E4/pull/114/files
Squish tests: https://github.com/ISISComputingGroup/System_Tests_UI_E4
GUI: https://github.com/ISISComputingGroup/ibex_gui (functionality mostly in uk.ac.stfc.isis.ibex.scriptgenerator.dynamicscripting)
Docs: https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/IBEX-Script-Generator-Dynamic-Scripting