* Use RimRafMode.UNLINK in tests to avoid filesystem contention (#114024)

* Add `rimrafWithRetries` for tests which incorrectly continue writing to the fixture directory even after the test signals completion

* 💄 No need to pass in RimRafMode.UNLINK since that is the default

* Also retry in case of EPERM

* Add some logging to check if rimraf really fails 5 times

* Fix retry logic

* Improve BackupTracker disposing to avoid fs writes after the test finishes

* Converted another ~70 test files to strictNullChecks

These are the remaining test files that didn't require any changes to non-test file types. I'll send those in separately.

Most changes were automatic and small (`!`s).
The only non-trival change is that `extHostSearch.ts` needs an interface for its test data.

Again used [TypeStat](https://github.com/JoshuaKGoldberg/TypeStat) when possible per #65581. I'm happy to split this up into smaller PRs if you'd prefer!

* Added missing `!`s to `commands.test.ts`

We set alwaysStrict so this directive is not needed