Use separate config dirs for kopia in file system mode (#4423)

Fixes possible issues of opening the
incorrect repo if tests are run in
parallel.

Integration test for this in
[model_store_test.go](3d78183651/src/internal/kopia/model_store_test.go (L897))

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #4422

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/pkg/storage/testdata/storage.go

@@ -68,6 +68,9 @@ func NewFilesystemStorage(t tester.TestT) storage.Storage {
 		},
 		storage.CommonConfig{
 			Corso: GetAndInsertCorso(""),
+			// Use separate kopia configs for each instance. Place in a new folder to
+			// avoid mixing data.
+			KopiaCfgDir: t.TempDir(),
 		})
 	require.NoError(t, err, "creating storage", clues.ToCore(err))