test: Fix race condition in TestGrafanaClient initialization
test: Fix race condition in TestGrafanaClient initialization

On Fedora/RHEL systems, starting pmproxy and redis in parallel does not
work: pmproxy needs to start *after* redis is already running, so that
its metrics API actually works. So use the same code structure on all
operating systems.

PR #15812 will fix that properly and get rid of that manual
initialization, but that still takes a while to land.

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