Screenshot tests fail when not run all together

[matthew-hagemann]

The screenshot tests in ubuntu_init and ubuntu_bootstrap can only be run as a group of tests. Running a single test will result in the test passing, but reporting back as a failure. The tests need to be updated such that they can be run individually:

For example, this is the test output from running the welcome test from ubuntu_init, which returns a failure:

...
Building Linux application...                                   
{"testID":1,"result":"success","skipped":false,"hidden":true,"type":"testDone","time":14337}
{"group":{"id":2,"suiteID":0,"parentID":null,"name":"","metadata":{"skip":false,"skipReason":null},"testCount":1,"line":null,"column":null,"url":null},"type":"group","time":14339}
{"test":{"id":3,"name":"(setUpAll)","suiteID":0,"groupIDs":[2],"metadata":{"skip":false,"skipReason":null},"line":14,"column":3,"url":"file:///home/matt/dev/flutter-get-gud/ubuntu-desktop-provision/packages/ubuntu
_init/integration_test/screenshot_test.dart"},"type":"testStart","time":14339}
{"testID":3,"result":"success","skipped":false,"hidden":true,"type":"testDone","time":14354}
{"test":{"id":4,"name":"welcome (variant: light)","suiteID":0,"groupIDs":[2],"metadata":{"skip":false,"skipReason":null},"line":165,"column":5,"url":"package:flutter_test/src/widget_tester.dart","root_line":20,"ro
ot_column":3,"root_url":"file:///home/matt/dev/flutter-get-gud/ubuntu-desktop-provision/packages/ubuntu_init/integration_test/screenshot_test.dart"},"type":"testStart","time":14354}
{"testID":4,"messageType":"print","message":"INFO ubuntu_init: Logging to /var/log/installer/ubuntu_init.log","type":"print","time":14586}
{"testID":4,"messageType":"print","message":"DEBUG ubuntu_init: Initializing YaruWindowTitleBar","type":"print","time":14587}
{"testID":4,"messageType":"print","message":"DEBUG ubuntu_init: Initializing services","type":"print","time":14594}
{"testID":4,"messageType":"print","message":"DEBUG ubuntu_init: Loading theme config","type":"print","time":15141}
{"testID":4,"messageType":"print","message":"DEBUG ubuntu_init: Loading page config","type":"print","time":15158}
{"testID":4,"messageType":"print","message":"INFO flavor_provider: Using default flavor: UbuntuFlavor.ubuntu","type":"print","time":15161}
{"testID":4,"result":"success","skipped":false,"hidden":false,"type":"testDone","time":16426}
{"test":{"id":5,"name":"(tearDownAll)","suiteID":0,"groupIDs":[2],"metadata":{"skip":false,"skipReason":null},"line":42,"column":5,"url":"package:integration_test/integration_test.dart"},"type":"testStart","time":
16426}
{"testID":5,"result":"success","skipped":false,"hidden":true,"type":"testDone","time":16432}
{"success":true,"type":"done","time":16439}