Reland "Switch from rtc_ios_xctest_test to rtc_test".

This is a reland of https://webrtc-review.googlesource.com/c/src/+/182680,
and it is based on https://chromium-review.googlesource.com/c/chromium/src/+/2414169
which allows to force a "test" to be executed as an XCtest on a build target
basis instead of on a GN args basis like it was happening in
https://webrtc-review.googlesource.com/c/src/+/182680 because that was
causing issues when launching tests on real devices.

Bug: webrtc:11881, webrtc:11937
Change-Id: I4bec7f471cd0f878b6050f266cd0cca3b518d415
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/184360
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32126}

examples/BUILD.gn

@@ -632,7 +632,8 @@ if (is_ios || (is_mac && target_cpu != "x86")) {
         ]
       }
 
-      rtc_ios_xctest_test("apprtcmobile_tests") {
+      rtc_test("apprtcmobile_tests") {
+        is_xctest = true
         info_plist = "objc/AppRTCMobile/ios/Info.plist"
         sources = [ "objc/AppRTCMobile/tests/main.mm" ]
         deps = [