fix tests

Author: paulb777

FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift

@@ -76,12 +76,15 @@ class AuthBackend {
     return "FirebaseAuth.iOS/\(FirebaseVersion()) \(GTMFetcherStandardUserAgentString(nil))"
   }
 
-  private static var gBackendImplementation = AuthBackendRPCImplementation()
+  private static var realRPCBackend = AuthBackendRPCImplementation()
+  private static var gBackendImplementation = realRPCBackend
 
-  class func setDefaultBackendImplementationWithRPCIssuer(issuer: AuthBackendRPCIssuer?) {
-    if let issuer = issuer {
-      gBackendImplementation.rpcIssuer = issuer
-    }
+  class func setTestRPCIssuer(issuer: AuthBackendRPCIssuer) {
+    gBackendImplementation.rpcIssuer = issuer
+  }
+
+  class func resetRPCIssuer() {
+    gBackendImplementation.rpcIssuer = realRPCBackend.rpcIssuer
   }
 
   class func implementation() -> AuthBackendImplementation {

FirebaseAuth/Tests/Unit/RPCBaseTests.swift

@@ -72,13 +72,13 @@ class RPCBaseTests: XCTestCase {
 
   override func setUp() {
     rpcIssuer = FakeBackendRPCIssuer()
-    AuthBackend.setDefaultBackendImplementationWithRPCIssuer(issuer: rpcIssuer)
+    AuthBackend.setTestRPCIssuer(issuer: rpcIssuer)
     rpcImplementation = AuthBackend.implementation()
   }
 
   override func tearDown() {
     rpcIssuer = nil
-    AuthBackend.setDefaultBackendImplementationWithRPCIssuer(issuer: nil)
+    AuthBackend.resetRPCIssuer()
   }
 
   /** @fn checkRequest