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, 14 insertions, 6 deletions

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);