Fix Firestore tests for M22 (#834)

* Add FIRFirestoreTests to the Firestore Xcode project

* Avoid waitForExpectations:timeout:

This API was added in Xcode 8.3, but we still build production releases
with Xcode 8.2. waitForExpectationsWithTimeout:handler: is available
from Xcode 7.2.

* Add AppForUnitTesting

Add a utility for constructing a Firebase App for testing.

* Handle the nil UID from FIRAuth

* Avoid running CMake tests twice

* Only build app_testing on Apple platforms

* Revise test.sh messages

1 files changed, 3 insertions, 2 deletions

diff --git a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
index 65c4c65..66c3c87 100644
--- a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
+++ b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
@@ -26,6 +26,7 @@
 
 #include <memory>
 #include <mutex>  // NOLINT(build/c++11)
+#include <utility>
 
 #include "Firestore/core/src/firebase/firestore/auth/credentials_provider.h"
 #include "Firestore/core/src/firebase/firestore/auth/user.h"
@@ -76,8 +77,8 @@ class FirebaseCredentialsProvider : public CredentialsProvider {
    * avoid races between notifications arriving and C++ object destruction.
    */
   struct Contents {
-    Contents(FIRApp* app, const absl::string_view uid)
-        : app(app), current_user(uid), mutex() {
+    Contents(FIRApp* app, User&& user)
+        : app(app), current_user(std::move(user)), mutex() {
     }
 
     const FIRApp* app;