Add xctest target to migrate sdk/objc/Framework to it.

This CL adds all the scaffolding needed to support xctest target
in sdk/objc/Framework and adds the target to the bot configuration.

The benefits of this are two-fold.
1. We'll separate framework unittests from their current target,
`rtc_unittests`, that has many many other tests.
This way framework unit tests will have nice, compact, selfcontained target.
2. We'll harvest the power of XCTest (native testing framework)
that should hopefully make adding and writing objc tests easier.

This CL migrates only one test to prove the setup works.
More tests will be migrated in follow up cls.

Bug: webrtc:8382
Change-Id: I0b5b9596c2a6d91683d13632323441de1aa461e0
Reviewed-on: https://webrtc-review.googlesource.com/8501
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Reviewed-by: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Commit-Queue: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20289}

tools_webrtc/ios/tests/common_tests.json

@@ -4,6 +4,10 @@
       "app": "apprtcmobile_tests",
       "xctest": true
     },
+    {
+       "app": "sdk_unittests",
+       "xctest": true
+    },
     {
       "app": "audio_decoder_unittests"
     },