Fix local tests (#20120)

## Summary of the Pull Request
Fixes TabTests.cpp. Changes include:
- add `"closeOnExit": "never"` to some tests to avoid the race condition
discussed in #14623. It keeps the tabs/panes around so that we can
actually test them.
- removes the `assert(false)` in `TermControl::MinimumSize()`. I've been
hitting this assert in dev builds _every time_ I split a pane. I think
it isn't adding value, but if we want to investigate it further, we can.
- update MRU tests to not use the command palette (aka tab switcher) UI.
The UI requires you to hold a modifier key down to keep it displayed.
That isn't something we can simulate. Instead we just use the relevant
APIs to simulate switching tabs. This is ok because the `MRU` list is
still updated, so we're still able to validate that we're updating it
properly.
- Add `Tab[idx]` and `MRU[idx]` notation to MRU tests to make them
easier to follow

## Validation Steps Performed
✅ local tests pass

Closes #19610
Closes #14623