test/browser: run tests inside the tasks container

This simplifies the "outside" setup quite a bit and gives us the same
tasks container that test runs on the Cockpit CI run under.

This change also implicitly changes the test browser to chromium (which
is no longer an issue — we just use it from the tasks container).

test/vm.install

@@ -1,9 +1,10 @@
 #!/bin/sh
 # image-customize script to prepare a bots VM for testing this application
 # The application package will be installed separately
-set -eu
+set -eux
 
 # don't force https:// (self-signed cert)
+mkdir -p /etc/cockpit
 printf "[WebService]\\nAllowUnencrypted=true\\n" > /etc/cockpit/cockpit.conf
 
 if type firewall-cmd >/dev/null 2>&1; then