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
    ```
    
    (cherry picked from commit 7fb4ee7aa5b6ffdf2e1e8e50a18630aa30f16505)