[ci] Add LUCI version of macOS custom tests (#2855)

This adds a bringup LUCI version of the macOS-host custom package tests.
This is the first attempt to migrate a flutter/packages or
flutter/plugins test to macOS ARM, so is experimental.

The motivation for doing this migration now is to see if it can unblock
https://github.com/flutter/packages/pull/2816, which doesn't seem to
be able to run successfully on Cirrus ARM machines (very possibly due
to the same issue that blocked flutter/plugins migrations to ARM on
Cirrus).

.ci.yaml

@@ -20,6 +20,17 @@ platform_properties:
         ]
       device_type: none
       os: Windows
+  mac_arm64:
+    properties:
+      dependencies: >-
+        [
+          {"dependency": "xcode", "version": "14a5294e"},
+          {"dependency": "gems", "version": "v3.3.14"}
+        ]
+      os: Mac-12
+      device_type: none
+      cpu: arm64
+      xcode: 14a5294e # xcode 14.0 beta 5
   mac_x64:
     properties:
       dependencies: >-
@@ -54,6 +65,26 @@ targets:
       target_file: mac_build_all_packages.yaml
       channel: stable
 
+  - name: Mac_arm64 custom_package_tests master
+    bringup: true # New configuration
+    recipe: packages/packages
+    timeout: 30
+    properties:
+      add_recipes_cq: "true"
+      version_file: flutter_master.version
+      target_file: mac_custom_package_tests.yaml
+      channel: master
+
+  - name: Mac_arm64 custom_package_tests stable
+    bringup: true # New configuration
+    recipe: packages/packages
+    timeout: 30
+    properties:
+      add_recipes_cq: "true"
+      version_file: flutter_stable.version
+      target_file: mac_custom_package_tests.yaml
+      channel: stable
+
   - name: Windows custom_package_tests master - packages
     recipe: packages/packages
     timeout: 30

.ci/targets/mac_custom_package_tests.yaml

@@ -0,0 +1,5 @@
+tasks:
+  - name: prepare tool
+    script: .ci/scripts/prepare_tool.sh
+  - name: custom package tests
+    script: .ci/scripts/custom_package_tests.sh