added support for multiple accounts with an account switcher in settings

test/api/cache_store_test.dart

@@ -18,4 +18,20 @@ void main() {
       expect(CacheStore.parse('abc\n{}'), isNull);
     });
   });
+
+  group('CacheStore.directoryName', () {
+    test('keeps the original directory for the pre-multi-account account', () {
+      expect(CacheStore.directoryName(''), 'request_cache');
+    });
+
+    test('separates accounts and the signed-out state', () {
+      final names = {
+        CacheStore.directoryName(''),
+        CacheStore.directoryName('a1b2'),
+        CacheStore.directoryName('c3d4'),
+        CacheStore.directoryName(null),
+      };
+      expect(names, hasLength(4));
+    });
+  });
 }