[CI][VSTS] Add support to run the tests on 10.12. (#10684)

tools/devops/automation/build-pipeline.yml

@@ -182,6 +182,7 @@ stages:
   parameters:
     stageName: 'Mac Mojave (10.14)'
     macPool: 'Hosted Mac Internal Mojave'
+    osVersion: '10.14'
     statusContext: 'Mac Mojave (10.14)'
     keyringPass: $(xma-password)
 
@@ -189,9 +190,18 @@ stages:
   parameters:
     stageName: 'Mac High Sierra (10.13)'
     macPool: 'Hosted Mac Internal'
+    osVersion: '10.13'
     statusContext: 'Mac High Sierra (10.13)'
     keyringPass: $(xma-password)
 
+- template: templates/mac/stage.yml
+  parameters:
+    stageName: 'Mac Sierra (10.12)'
+    macPool: 'VSEng-Xamarin-RedmondMacTestPool-iOS'
+    osVersion: '10.12'
+    statusContext: 'Mac Sierra (10.12)'
+    keyringPass: $(xma-password)
+
 # TODO: Not the real step
 - stage: sample_testing
   displayName: 'Sample testing'

tools/devops/automation/templates/mac/stage.yml

@@ -8,11 +8,14 @@ parameters:
 
 - name: statusContext
   type: string 
-  default: '10.16' # default context, since we started dealing with iOS devices. 
+  default: '10.16'
 
 - name: keyringPass
   type: string
 
+- name: osVersion
+  type: string
+
 stages:
 - stage:
   displayName: ${{ parameters.stageName }}
@@ -32,6 +35,7 @@ stages:
       name: ${{ parameters.macPool }}
       demands: 
       - Agent.OS -equals Darwin
+      - Agent.OSVersion -equals ${{ parameters.osVersion }}
 
     steps:
     - template: build.yml