Remove objc tests from rtc_unittests and move tests to sdk_unittests

The tests in rtc_unittests_objc are old gtest based tests and
The tests in sdk_unittests are XCTest based tests.
The objc tests in rtc_unittest are causing problems [1],
so I think it's time to combine the two types of objc tests.

Renaming the files to match the existing sdk_unittests and
removing the use of gtest helper functions (eg, EXPECT_EQ)
are planned for follow-up CLs.

[1] https://webrtc-review.googlesource.com/c/src/+/261724/9

Bug: webrtc:8382
Change-Id: I9308b35f654c4479c9df72bf7cd7f4032e267eb2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262380
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Daniel.L (Byoungchan) Lee <daniel.l@hpcnt.com>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36878}

sdk/objc/unittests/RTCCertificateTest.mm

@@ -9,6 +9,7 @@
  */
 
 #import <Foundation/Foundation.h>
+#import <XCTest/XCTest.h>
 
 #include <vector>
 
@@ -22,8 +23,7 @@
 #import "api/peerconnection/RTCPeerConnectionFactory.h"
 #import "helpers/NSString+StdString.h"
 
-@interface RTCCertificateTest : NSObject
-- (void)testCertificateIsUsedInConfig;
+@interface RTCCertificateTest : XCTestCase
 @end
 
 @implementation RTCCertificateTest
@@ -71,8 +71,3 @@
 }
 
 @end
-
-TEST(CertificateTest, DISABLED_CertificateIsUsedInConfig) {
-  RTCCertificateTest *test = [[RTCCertificateTest alloc] init];
-  [test testCertificateIsUsedInConfig];
-}