systemd: Don't race with timedated proxy initialization
systemd: Don't race with timedated proxy initialization

Since we don't wait for the timedated proxy to initialize,
just use the proxy.call() method to invoke methods.

This fixes a race in the tests.

Closes #2767

arilivigni/cockpit,sgallagher/cockpit,michalskrivanek/cockpit,Scribery/cockpit,SotolitoLabs/cockpit,petervo/cockpit,vanloswang/cockpit,vanloswang/cockpit,harishanand95/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,vanloswang/cockpit,petervo/cockpit,zigitax/cockpit,zigitax/cockpit,zigitax/cockpit,haiyangd/cockpit_view,cockpit-project/cockpit,michalskrivanek/cockpit,vanloswang/cockpit,jscotka/cockpit,harishanand95/cockpit,cockpituous/cockpit,Scribery/cockpit,Scribery/cockpit,mareklibra/cockpit,sgallagher/cockpit,mareklibra/cockpit,dperpeet/cockpit,mareklibra/cockpit,moolitayer/cockpit,andreasn/cockpit,martinpitt/cockpit,FireDrunk/cockpit,matobet/cockpit,michalskrivanek/cockpit,FireDrunk/cockpit,sgallagher/cockpit,dperpeet/cockpit,xhad/cockpit,andreasn/cockpit,mvollmer/cockpit,mareklibra/cockpit,haiyangd/cockpit_view,haiyangd/cockpit_view,moolitayer/cockpit,stefwalter/cockpit,cockpituous/cockpit,xhad/cockpit,SotolitoLabs/cockpit,petervo/cockpit,SotolitoLabs/cockpit,larskarlitski/cockpit,mvollmer/cockpit,Scribery/cockpit,Scribery/cockpit,jscotka/cockpit,andreasn/cockpit,harishanand95/cockpit,larsu/cockpit,michalskrivanek/cockpit,moraleslazaro/cockpit,sgallagher/cockpit,haiyangd/cockpit_view,larskarlitski/cockpit,michalskrivanek/cockpit,moraleslazaro/cockpit,matobet/cockpit,stefwalter/cockpit,arilivigni/cockpit,mvollmer/cockpit,garrett/cockpit,larskarlitski/cockpit,mvollmer/cockpit,matobet/cockpit,matobet/cockpit,cockpit-project/cockpit,mareklibra/cockpit,FireDrunk/cockpit,larsu/cockpit,zigitax/cockpit,martinpitt/cockpit,cockpituous/cockpit,larsu/cockpit,moraleslazaro/cockpit,larskarlitski/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,deryni/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,dperpeet/cockpit,xhad/cockpit,dperpeet/cockpit,moraleslazaro/cockpit,deryni/cockpit,petervo/cockpit,FireDrunk/cockpit,deryni/cockpit,FireDrunk/cockpit,SotolitoLabs/cockpit,xhad/cockpit,Scribery/cockpit,jscotka/cockpit,larskarlitski/cockpit,zigitax/cockpit,moolitayer/cockpit,andreasn/cockpit,andreasn/cockpit,arilivigni/cockpit,matobet/cockpit,michalskrivanek/cockpit,SotolitoLabs/cockpit,garrett/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,arilivigni/cockpit,harishanand95/cockpit,stefwalter/cockpit,Scribery/cockpit,mareklibra/cockpit,vanloswang/cockpit,haiyangd/cockpit_view,martinpitt/cockpit,garrett/cockpit,jscotka/cockpit,garrett/cockpit,dperpeet/cockpit,larskarlitski/cockpit,moraleslazaro/cockpit,arilivigni/cockpit,arilivigni/cockpit,martinpitt/cockpit,cockpituous/cockpit,michalskrivanek/cockpit,stefwalter/cockpit,petervo/cockpit,harishanand95/cockpit,xhad/cockpit,petervo/cockpit,sgallagher/cockpit,cockpituous/cockpit,mareklibra/cockpit,jscotka/cockpit,cockpit-project/cockpit,dperpeet/cockpit,xhad/cockpit,vanloswang/cockpit,deryni/cockpit,harishanand95/cockpit,deryni/cockpit,arilivigni/cockpit,petervo/cockpit,vanloswang/cockpit,moolitayer/cockpit,harishanand95/cockpit,larsu/cockpit,deryni/cockpit,mvollmer/cockpit,cockpituous/cockpit,moolitayer/cockpit,larsu/cockpit,stefwalter/cockpit,xhad/cockpit,dperpeet/cockpit,sgallagher/cockpit,sgallagher/cockpit,moraleslazaro/cockpit,larskarlitski/cockpit,larsu/cockpit,stefwalter/cockpit,jscotka/cockpit,garrett/cockpit,deryni/cockpit,matobet/cockpit