Bug 1602863 - run desktop-screenshot-capture tests with ubuntu1804 docker image r=jmaher

Changes:

Run mochitest-screenshots with ubuntu1804, and remove from running under `linux64` label.

Also update the references in `mochitest.yml` for the `run-on-projects` scheduler.

Differential Revision: https://phabricator.services.mozilla.com/D58864

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/mochitest.yml

@@ -160,8 +160,8 @@ browser-screenshots:
         by-test-platform:
             windows7-32(?:-pgo|-shippable)(?:-qr)?/opt: ['mozilla-central', 'integration']
             windows10-64(?:-pgo|-shippable)(?:-qr)?/opt: ['mozilla-central', 'integration']
-            (?:windows10-64|windows7-32|linux64)(?:-qr)?/opt: ['mozilla-central']
-            linux64-(?:pgo|shippable)(?:-qr)?/opt: ['mozilla-central', 'integration']
+            (?:windows10-64|windows7-32|linux1804-64)(?:-qr)?/opt: ['mozilla-central']
+            linux1804-64-(?:pgo|shippable)(?:-qr)?/opt: ['mozilla-central', 'integration']
             macosx.*64-shippable/opt: ['mozilla-central', 'integration']
             default: []
     fission-run-on-projects: []

taskcluster/ci/test/test-platforms.yml

@@ -28,6 +28,7 @@ linux1804-64/opt:
     build-platform: linux64/opt
     test-sets:
         - awsy
+        - desktop-screenshot-capture
         - linux1804-tests
 
 linux1804-64/debug:
@@ -39,12 +40,14 @@ linux1804-64-shippable/opt:
     build-platform: linux64-shippable/opt
     test-sets:
         - awsy
+        - desktop-screenshot-capture
         - linux1804-tests
         - marionette-headless
 
 linux1804-64-devedition/opt:
     build-platform: linux64-devedition-nightly/opt
     test-sets:
+        - desktop-screenshot-capture
         - linux1804-tests
 
 linux1804-64-asan/opt:
@@ -99,7 +102,6 @@ linux64/opt:
         - linux-common-tests
         - web-platform-tests
         - opt-only-tests
-        - desktop-screenshot-capture
         - talos
         - mochitest-headless
         - linux-talos-flex
@@ -114,7 +116,6 @@ linux64-shippable/opt:
         - web-platform-tests
         - web-platform-tests-wdspec-headless
         - opt-only-tests
-        - desktop-screenshot-capture
         - talos
         - mochitest-headless
         - linux-talos-flex
@@ -134,7 +135,6 @@ linux64-devedition/opt:
         - linux-common-tests
         - web-platform-tests
         - opt-only-tests
-        - desktop-screenshot-capture
 
 # TODO: use 'asan' label here, instead of -asan/opt
 linux64-asan/opt: