bridge: Fix fd passing test
bridge: Fix fd passing test

The "receive readable fd and ensure writing fails" test in
dbus-common.js could fail when the bridge closed the CockpitPipe
associated with the fd before the test tries the write. In that case,
the bridge doesn't return a "protocol-error", but a more generic one
about the channel not existing anymore.

Keep the writable side of the fd open for a bit, so that this test
actually tests that writing to that fd fails.

Closes #8453
stefwalter/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,petervo/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,stefwalter/cockpit,cockpit-project/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,andreasn/cockpit,cockpituous/cockpit,andreasn/cockpit,mareklibra/cockpit,deryni/cockpit,mareklibra/cockpit,petervo/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,mareklibra/cockpit,deryni/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,mvollmer/cockpit,mareklibra/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,petervo/cockpit,petervo/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,cockpit-project/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,deryni/cockpit,mvollmer/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,deryni/cockpit,stefwalter/cockpit,andreasn/cockpit,cockpituous/cockpit,deryni/cockpit,mareklibra/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,andreasn/cockpit,petervo/cockpit,garrett/cockpit,stefwalter/cockpit,andreasn/cockpit,andreasn/cockpit,cockpit-project/cockpit,deryni/cockpit,petervo/cockpit,mareklibra/cockpit,andreasn/cockpit,moraleslazaro/cockpit,garrett/cockpit,petervo/cockpit