Block deprecated warnings when testing on newer OS

Some of the test methods we use are deprecated on newer OS. This just gets prevents the
warning that we don't actually care about.

objectivec/Tests/GPBMessageTests.m

@@ -383,6 +383,9 @@ - (void)testParseUninitialized {
 #endif  // DEBUG
 }
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+
 - (void)testCoding {
   GPBMessage *original = [self mergeResult];
   NSData *data =
@@ -418,6 +421,8 @@ - (void)testSecureCoding {
   XCTAssertNotEqual(unarchivedObject, original);
 }
 
+#pragma clang diagnostic pop
+
 - (void)testObjectReset {
   // Tests a failure where clearing out defaults values caused an over release.
   TestAllTypes *message = [TestAllTypes message];