workflows: Fix race between flatpak tests
workflows: Fix race between flatpak tests

Even after commit 91854c8cc7b492, test-browser still sometimes tries to
talk to an allegedly existing instance, and then fails on talking into
the void. There are no related processes any more, apparently the
session D-Bus does not recognize the disappeared process fast enough.

Let's avoid that by running each test in its own D-Bus session.

Note: Keep sharing a single Xvfb server, as xvfb-run does not like being
run multiple times in a row without waiting.

mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit