[ios/infra] move xcode-parallelization to testing/buildbot

Not all EG2 tests run with xcode-parallelization. This is an arg
that's primiarily used with simulators. Downstream, eg2 tests
are also run on devices.

Because simulators are defined in testing/buildbot, and because
the xcode-parallelization arg is bound to platform and version
(both defined by simulators), we move this arg down to testing.

* Introduced support for mixins one level above variants such that
a mixin defined here applies to all variants below.
* Removed the --xcode-parallelization arg in the GN EG2 iOS
template.
* Added xcode-parallelization for all test targets that ran
through the eg2 target upstream.

Bug: 912681
Change-Id: Ie5cb45a14be0ecbb6fb64b7c8af6eea6f49df94c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364031
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800180}
GitOrigin-RevId: 85fb8df06132058d3dce41614e076d6ac5ee6116

config/ios/ios_test_runner_wrapper.gni

@@ -134,6 +134,7 @@ template("ios_test_runner_wrapper") {
       "//testing/test_env.py",
       "//.vpython",
       "//ios/build/bots/scripts/",
+      "//testing/test_env.py",
     ]
   }
 }