Revert "Disable Facebook API tests (#2547)" (#2628)

This reverts commit ed2b99a.

Author: Chuan Ren

Example/Auth/ApiTests/FacebookAuthTests.m

@@ -36,7 +36,7 @@ @interface FacebookAuthTests : FIRAuthApiTestsBase
 
 @implementation FacebookAuthTests
 
-- (void)DISABLE_testSignInWithFaceboook {
+- (void)testSignInWithFaceboook {
   FIRAuth *auth = [FIRAuth auth];
   if (!auth) {
     XCTFail(@"Could not obtain auth object.");
@@ -76,7 +76,7 @@ - (void)DISABLE_testSignInWithFaceboook {
   [self deleteFacebookTestingAccountbyId:facebookAccountId];
 }
 
-- (void)DISABLE_testLinkAnonymousAccountToFacebookAccount {
+- (void)testLinkAnonymousAccountToFacebookAccount {
   FIRAuth *auth = [FIRAuth auth];
   if (!auth) {
     XCTFail(@"Could not obtain auth object.");