[ios] pass xctest for iOS unit tests

Passing xctest to the iOS test runner (run.py) for unittests to run with
the xc test runner object. This requires the GN argument
enable_run_ios_unittests_with_xctest to be True.

Bug: 1001667
Change-Id: Ib7f14c5835dbe4a116f11074646704bee8754d02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314134Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Jeff Yoon <jeffyoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791954}

testing/test.gni

@@ -274,6 +274,10 @@ template("test") {
         "@WrappedPath(${_root_build_dir}/${_test_target}.app)",
       ]
 
+      if (enable_run_ios_unittests_with_xctest) {
+        executable_args += [ "--xctest" ]
+      }
+
       wrapper_output_name = "${_wrapper_output_name}"
     }