test: Fix race condition in machines.json migration test (#6007)
test: Fix race condition in machines.json migration test (#6007)

The migration happens asynchronously after login in the privileged
bridge; we can't assume it's already done after logging in and the
dashboard shows the expected machines, as these may still have been read
from the old file in /var. Thus wait until the /var file is gone, or for
getting the error about the failed migration in the "nonwritable" case.
deryni/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,harishanand95/cockpit,larskarlitski/cockpit,mareklibra/cockpit,michalskrivanek/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpituous/cockpit,dperpeet/cockpit,larsu/cockpit,xhad/cockpit,Scribery/cockpit,moraleslazaro/cockpit,xhad/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,dperpeet/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,mvollmer/cockpit,stefwalter/cockpit,dperpeet/cockpit,cockpituous/cockpit,cockpit-project/cockpit,xhad/cockpit,andreasn/cockpit,garrett/cockpit,SotolitoLabs/cockpit,petervo/cockpit,cockpit-project/cockpit,mvollmer/cockpit,harishanand95/cockpit,harishanand95/cockpit,martinpitt/cockpit,xhad/cockpit,Scribery/cockpit,deryni/cockpit,cockpit-project/cockpit,xhad/cockpit,harishanand95/cockpit,dperpeet/cockpit,larsu/cockpit,mareklibra/cockpit,xhad/cockpit,Scribery/cockpit,dperpeet/cockpit,petervo/cockpit,larsu/cockpit,michalskrivanek/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,petervo/cockpit,larskarlitski/cockpit,martinpitt/cockpit,garrett/cockpit,michalskrivanek/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,mareklibra/cockpit,mareklibra/cockpit,deryni/cockpit,mvollmer/cockpit,michalskrivanek/cockpit,larskarlitski/cockpit,larsu/cockpit,deryni/cockpit,larsu/cockpit,stefwalter/cockpit,cockpit-project/cockpit,harishanand95/cockpit,xhad/cockpit,stefwalter/cockpit,garrett/cockpit,michalskrivanek/cockpit,andreasn/cockpit,martinpitt/cockpit,petervo/cockpit,petervo/cockpit,stefwalter/cockpit,petervo/cockpit,cockpituous/cockpit,mareklibra/cockpit,larskarlitski/cockpit,cockpituous/cockpit,cockpituous/cockpit,andreasn/cockpit,dperpeet/cockpit,stefwalter/cockpit,harishanand95/cockpit,mareklibra/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,larskarlitski/cockpit,Scribery/cockpit,moraleslazaro/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,larskarlitski/cockpit,andreasn/cockpit,Scribery/cockpit,michalskrivanek/cockpit,dperpeet/cockpit,larsu/cockpit,deryni/cockpit,SotolitoLabs/cockpit,larskarlitski/cockpit,stefwalter/cockpit,andreasn/cockpit,petervo/cockpit,harishanand95/cockpit,michalskrivanek/cockpit,garrett/cockpit,mareklibra/cockpit,andreasn/cockpit