test: Generate screenshot artifact of main screen

This is a robust screenshot of a steady state without expected jitter.

This is useful for visually validating PatternFly update PRs without
having to locally build and run. In the future we can also run
programmatic image diffs on these to auto-detect pixel
changes/layout regressions.

test/check-application

@@ -25,6 +25,7 @@ class TestApplication(testlib.MachineCase):
         # verify expected host name
         hostname = m.execute("hostname").strip()
         b.wait_in_text(".pf-c-alert__title", "Running on " + hostname)
+        b.snapshot("main-english")
 
         # change current hostname
         m.execute("echo new-%s > /etc/hostname" % hostname)