test: Fix test-dbus to work without a system bus (#7952)
test: Fix test-dbus to work without a system bus (#7952)

The last test in `TESTS=dist/base1/test-dbus` hangs eternally if there
is no system D-Bus running:

```
PASS: dist/base1/test-dbus.html 187 - receive writable fd: fd received and not writable
cockpit-bridge-Message: 1:58: Could not connect: No such file or directory
cockpit-bridge-Message: 1:60: (null)
cockpit-bridge-Message: 1:59: Could not connect: No such file or directory
cockpit-bridge-Message: 1:61: Could not connect: No such file or directory
Could not connect: No such file or directory
(null)
Could not connect: No such file or directory
Could not connect: No such file or directory
```

This happens when running tests in mock or in a docker container.

Use the session bus to avoid this and make fewer assumptions about the
host system.

Closes #7952
garrett/cockpit,petervo/cockpit,stefwalter/cockpit,mareklibra/cockpit,petervo/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,cockpituous/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,petervo/cockpit,deryni/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,stefwalter/cockpit,mareklibra/cockpit,cockpit-project/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,andreasn/cockpit,mareklibra/cockpit,martinpitt/cockpit,martinpitt/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,Scribery/cockpit,petervo/cockpit,petervo/cockpit,petervo/cockpit,moraleslazaro/cockpit,andreasn/cockpit,martinpitt/cockpit,cockpituous/cockpit,Scribery/cockpit,mvollmer/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,stefwalter/cockpit,garrett/cockpit,Scribery/cockpit,cockpit-project/cockpit,mareklibra/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,cockpit-project/cockpit,petervo/cockpit,mvollmer/cockpit,stefwalter/cockpit,stefwalter/cockpit,stefwalter/cockpit,deryni/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,deryni/cockpit,Scribery/cockpit,andreasn/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,Scribery/cockpit,andreasn/cockpit