ci: use templates for VSTS builds

Our build YAML is becoming unweildly and full of copy-pasta.  Simplify
with templates.

ci/vsts-bash.yml

+# These are the steps used for building on machines with bash.
+steps:
+- bash: . '$(Build.SourcesDirectory)/ci/build.sh'
+  displayName: Build
+  workingDirectory: '$(Build.BinariesDirectory)'
+  env: ${{ parameters.environmentVariables }}
+- bash: . '$(Build.SourcesDirectory)/ci/test.sh'
+  displayName: Test
+  workingDirectory: '$(Build.BinariesDirectory)'
+  env: ${{ parameters.environmentVariables }}
+- task: PublishTestResults@2
+  displayName: Publish Test Results
+  condition: succeededOrFailed()
+  inputs:
+    testResultsFiles: 'results_*.xml'
+    searchFolder: '$(Build.BinariesDirectory)'
+    mergeTestResults: true

ci/vsts-docker.yml

+# These are the steps used in a container-based build in VSTS.
+steps:
+- task: docker@0
+  displayName: Build
+  inputs:
+    action: 'Run an image'
+    imageName: ${{ parameters.imageName }}
+    volumes: |
+     $(Build.SourcesDirectory):/src
+     $(Build.BinariesDirectory):/build
+    envVars: ${{ parameters.environmentVariables }}
+    workDir: '/build'
+    containerCommand: '/src/ci/build.sh'
+    detached: false
+- task: docker@0
+  displayName: Test
+  inputs:
+    action: 'Run an image'
+    imageName: ${{ parameters.imageName }}
+    volumes: |
+     $(Build.SourcesDirectory):/src
+     $(Build.BinariesDirectory):/build
+    envVars: ${{ parameters.environmentVariables }}
+    workDir: '/build'
+    containerCommand: '/src/ci/test.sh'
+    detached: false
+- task: publishtestresults@2
+  displayName: Publish Test Results
+  condition: succeededOrFailed()
+  inputs:
+    testResultsFiles: 'results_*.xml'
+    searchFolder: '$(Build.BinariesDirectory)'
+    mergeTestResults: true

ci/vsts-powershell.yml

+# These are the steps used for building on machines with PowerShell.
+steps:
+- powershell: . '$(Build.SourcesDirectory)\ci\build.ps1'
+  displayName: Build
+  workingDirectory: '$(Build.BinariesDirectory)'
+  env: ${{ parameters.environmentVariables }}
+- powershell: . '$(Build.SourcesDirectory)\ci\test.ps1'
+  displayName: Test
+  workingDirectory: '$(Build.BinariesDirectory)'
+  env: ${{ parameters.environmentVariables }}
+- task: PublishTestResults@2
+  displayName: Publish Test Results
+  condition: succeededOrFailed()
+  inputs:
+    testResultsFiles: 'results_*.xml'
+    searchFolder: '$(Build.BinariesDirectory)'
+    mergeTestResults: true