* Make all FIRLogger functions extern "C"
* Make FSTIntegrationTestCase Objective-C++
Keep external interface to FSTIntegrationTestCase Objective-C
* Add explicit dependency on Firebase/Community/Core from Tests
This fixes a linkage error on a missing _FIRSetLoggerLevel symbol
* Use GoogleTest as a dependency of Firestore_Tests
* Remove top-level leveldb-library from HEADER_SEARCH_PATHS
* Add string_util_test to the project and get it to build
* Implement FSTGoogleTestTests, a bridge between GoogleTest and XCTest