test: bump the test timeout to give ldconfig.service enough time to finish

Sometimes the ldconfig.service might take a bit longer to finish,
causing spurious test timeouts:

```
[ 1025.858923] systemd[24]: ldconfig.service: Executing: /sbin/ldconfig -X
...
[ 1043.883620] systemd[1]: ldconfig.service: Main process exited, code=exited, status=0/SUCCESS (success)
...
Trying to halt container. Send SIGTERM again to trigger immediate
termination.
Container TEST-52-HONORFIRSTSHUTDOWN terminated by signal KILL.
E: Test timed out after 20s
```