Enable Analytics OSS Tests (Desktop)

Moved analytics test files into tests/ folder, and added CMakeLists.txt file to build tests.

PiperOrigin-RevId: 258970735

Author: ryanmeier

analytics/CMakeLists.txt

@@ -146,3 +146,8 @@ if(IOS)
   # Add a dependency to downloading the headers onto analytics.
   add_dependencies(firebase_analytics ${pod_target_name})
 endif()
+
+if(FIREBASE_CPP_BUILD_TESTS)
+  # Add the tests subdirectory
+  add_subdirectory(tests)
+endif()