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

3 files changed, 18 insertions, 10 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;
diff --git a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm
index f463958..fe3cb24 100644
--- a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm
+++ b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm
@@ -28,8 +28,7 @@ namespace firestore {
 namespace auth {
 
 FirebaseCredentialsProvider::FirebaseCredentialsProvider(FIRApp* app)
-    : contents_(
-          std::make_shared<Contents>(app, util::MakeStringView([app getUID]))) {
+    : contents_(std::make_shared<Contents>(app, User::FromUid([app getUID]))) {
   std::weak_ptr<Contents> weak_contents = contents_;
 
   auth_listener_handle_ = [[NSNotificationCenter defaultCenter]
diff --git a/Firestore/core/src/firebase/firestore/auth/user.h b/Firestore/core/src/firebase/firestore/auth/user.h
index cc7b66d..3918c61 100644
--- a/Firestore/core/src/firebase/firestore/auth/user.h
+++ b/Firestore/core/src/firebase/firestore/auth/user.h
@@ -43,12 +43,6 @@ class User {
   /** Construct an authenticated user with the given UID. */
   explicit User(const absl::string_view uid);
 
-#if defined(__OBJC__)
-  explicit User(NSString* uid)
-      : User(firebase::firestore::util::MakeStringView(uid)) {
-  }
-#endif  // defined(__OBJC__)
-
   const std::string& uid() const {
     return uid_;
   }
@@ -62,6 +56,20 @@ class User {
   /** Returns an unauthenticated instance. */
   static const User& Unauthenticated();
 
+#if defined(__OBJC__)
+  /**
+   * Returns an authenticated user if uid is non-nil, otherwise an
+   * unauthenticated user.
+   */
+  static User FromUid(NSString* _Nullable uid) {
+    if (uid == nil) {
+      return Unauthenticated();
+    } else {
+      return User(util::MakeStringView(uid));
+    }
+  }
+#endif  // defined(__OBJC__)
+
   User& operator=(const User& other) = default;
 
   friend bool operator==(const User& lhs, const User& rhs);