test: disable popup blocking for Cockpit-files

In Cockpit-files we download using `window.open` which Chromium
sometimes blocks depending on how long the browser was open, the session
was active or other unknown heuristics. To stop the tests from flaking
disable popup blocking in general as this also allows a Developer to now
run `TestFiles.testDownload` as a separate test without getting
window.open blocked.

test/common/cdp.py

@@ -105,6 +105,8 @@ def cmd(
                 "--disable-namespace-sandbox", "--disable-seccomp-filter-sandbox",
                 "--disable-sandbox-denial-logging", "--disable-pushstate-throttle",
                 "--font-render-hinting=none",
+                # HACK: For Cockpit-Files downloading uses `window.open` which is sometimes allowed depending on unpredictable and unknown heuristics
+                "--disable-popup-blocking",
                 "--v=0", f"--remote-debugging-port={cdp_port}", "about:blank"]