There are many specs in `ts_plugin_spec.ts` that exercise the behavior
of completions. These specs should belong in `completions_spec` instead.
In addition,
1. Tests for `getExternalFiles()` added in `ts_plugin_spec.ts`
2. Fixed bug in MockHost.reset() to remove overriden script names
3. Add test for TS diagnostics when `angularOnly = true` is not set
PR Close#33159