test: Disable parallel job execution for container/kubernetes
test: Disable parallel job execution for container/kubernetes

Apparently running full chromium under xvfb is rather brittle (see
commit 9a6fce4e2eebd), and it's currently way too hard to get the
container/kubernetes test to pass. Disable the default parallelism
through `$TEST_JOBS` for the time being until headless chrome has a
solution for <https://bugs.chromium.org/p/chromium/issues/detail?id=757181>.

Issue #8284 tracks a more proper fix for this.

Closes #8282

petervo/cockpit,garrett/cockpit,andreasn/cockpit,cockpit-project/cockpit,Scribery/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,andreasn/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,mvollmer/cockpit,Scribery/cockpit,martinpitt/cockpit,stefwalter/cockpit,petervo/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,petervo/cockpit,moraleslazaro/cockpit,Scribery/cockpit,Scribery/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpituous/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,petervo/cockpit,andreasn/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,petervo/cockpit,mareklibra/cockpit,cockpituous/cockpit,stefwalter/cockpit,deryni/cockpit,Scribery/cockpit,cockpituous/cockpit,stefwalter/cockpit,deryni/cockpit,martinpitt/cockpit,petervo/cockpit,moraleslazaro/cockpit,stefwalter/cockpit,petervo/cockpit,andreasn/cockpit,cockpituous/cockpit,garrett/cockpit,mareklibra/cockpit,cockpit-project/cockpit,stefwalter/cockpit,mvollmer/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,stefwalter/cockpit,deryni/cockpit,mareklibra/cockpit,garrett/cockpit,deryni/cockpit,cockpit-project/cockpit,Scribery/cockpit,andreasn/cockpit,martinpitt/cockpit,garrett/cockpit,andreasn/cockpit,deryni/cockpit,SotolitoLabs/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,mareklibra/cockpit,andreasn/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,deryni/cockpit,garrett/cockpit,mvollmer/cockpit