Make sure to use signIn within test

pod lib lint fails with a warning due to the unused variable

Author: mdmathias

GoogleSignIn/Tests/Unit/GIDSignInTest.m

@@ -1000,6 +1000,7 @@ - (void)testFreshInstall_removesKeychainEntries {
   GIDSignIn *signIn = [[GIDSignIn alloc] initWithKeychainStore:_keychainStore
                                      authStateMigrationService:_authStateMigrationService];
   // If `isFreshInstall`, keychain entries should be removed.
+  XCTAssertNotNil(signIn);
   XCTAssertTrue(self->_keychainRemoved);
 }