Tabs API: better integration test coverage

[bpasero]

Testing #145585

I forgot to report this yesterday: I felt that a lot of the issues uncovered could have been found with just providing a very good integration test suite (see https://github.com/microsoft/vscode/issues/145773#issuecomment-1075967255 for example).

The tabs API is super nice to test actually.

A few things to ensure to cover:

• all events from tabGroups
• all properties of TabGroup and Tab before and after mutation
• mutations
  • opening editors
  • closing editors
  • moving editors
  • opening groups
  • closing groups
  • moving groups
  • making an editor active that was not active
  • making a group active that was not active
  • changing tab labels (e.g. in an untitled file changing first line)
  • pinning / unpinning
  • preview / sticky
  • dirty / saved
  • all the kinds (text, text diff, notebook, notebook diff, custom, unknown)
• special cases: workbench.editor.closeEmptyGroups: false and empty groups

We need to make sure that proper commands or API is used when altering state to prevent flaky tests as outlined in https://github.com/microsoft/vscode/issues/145773#issuecomment-1077782847

[lramos15]

Isn't this actually more difficult than we thought as @jrieken pointed out without tabs API supporting modification. Any commands we use have the opportunity to cause flakiness if they don't accept an identifier. Unless we just want to test with like a single editor for example so the action is always unambiguous.

[bpasero]

I would try to get as much coverage done as possible with the API you have and leave the rest for later. Via API you can open editors, make them dirty, even open multiple editor groups, so a bit of coverage should be possible.