Bug 1590304 [wpt PR 19820] - [Azure Pipelines] upgrade to Ubuntu 18.04 (from 16.04), a=testonly

Automatic update from web-platform-tests
[Azure Pipelines] upgrade to Ubuntu 18.04 (from 16.04)

These jobs only run `./wpt test-jobs` and `curl`, but it's just as
well to upgrade them well before Ubuntu 16.04 is no longer supported.

--

wpt-commits: 0740bf536a0dbdb60ef76dd0cb15980ebcc86c7d
wpt-pr: 19820

testing/web-platform/tests/.azure-pipelines.yml

@@ -60,7 +60,7 @@ jobs:
   displayName: './wpt test-jobs'
   condition: eq(variables['Build.Reason'], 'PullRequest')
   pool:
-    vmImage: 'ubuntu-16.04'
+    vmImage: 'ubuntu-18.04'
   steps:
   - template: tools/ci/azure/checkout.yml
   - script: |

testing/web-platform/tests/tools/ci/azure/fyi_hook.yml

@@ -10,7 +10,7 @@ jobs:
   displayName: 'wpt.fyi hook: ${{ parameters.artifactName }}'
   dependsOn: ${{ parameters.dependsOn }}
   pool:
-    vmImage: 'ubuntu-16.04'
+    vmImage: 'ubuntu-18.04'
   steps:
   - checkout: none
   - script: curl -f -s -S -d "artifact=${{ parameters.artifactName }}" -X POST https://wpt.fyi/api/checks/azure/$(Build.BuildId)