Enable Auth OSS tests

Moved auth test files into tests/ folder, added CMakeLists.txt files, and tweaked a few other files to enable Auth OSS tests in the open source build.

PiperOrigin-RevId: 258645801

Author: ryanmeier

auth/CMakeLists.txt

@@ -220,3 +220,8 @@ if(IOS)
   # Add a dependency to downloading the headers onto auth.
   add_dependencies(firebase_auth ${pod_target_name})
 endif()
+
+if(FIREBASE_CPP_BUILD_TESTS)
+  # Add the tests subdirectory
+  add_subdirectory(tests)
+endif()

testing/config_desktop.cc

@@ -30,6 +30,7 @@ const ConfigRow* ConfigGet(const char* fake) {
   if (g_test_data_config == nullptr) {
     ADD_FAILURE() << "No test data at all";
     assert(false);
+    return nullptr;
   }
 
   const TestDataConfig* config = GetTestDataConfig(g_test_data_config);