Tabs API: better integration test coverage

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