QA: Fix browser tests on cached anonymous pages

Older pages use flow-ui-button rather than mw-ui-button
Broken since build 52
https://integration.wikimedia.org/ci/view/
	BrowserTests/job/browsertests-Flow-en.wikipedia.beta.wmflabs.org-linux-chrome-sauce/

Update selector so we target both cached and non-cached cases.

Change-Id: I8d27685866068f5e7a1316727f1a5fcf32860fc1
Merge "QA: Fix browser tests on cached anonymous pages"