Use separate config dirs for kopia

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

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))