d/tests: run as root to fix debci

Current output:

    autopkgtest [00:48:24]: test loopback: [-----------------------
    Creating database for interface "lo"...
    Error: Unable to open database "/var/lib/vnstat/lo" for writing: Permission denied
    Error: Unable to read database "/var/lib/vnstat/lo": No such file or directory
    Error: Unable to read database "/var/lib/vnstat/lo": No such file or directory
    Error: Unable to read database "/var/lib/vnstat/lo": No such file or directory
    autopkgtest [00:48:24]: test loopback: -----------------------]
    autopkgtest [00:48:24]: test loopback:  - - - - - - - - - - results - - - - - - - - - -
    loopback             FAIL non-zero exit status 1
    autopkgtest [00:48:24]: @@@@@@@@@@@@@@@@@@@@ summary
    version              PASS
    loopback             FAIL non-zero exit status 1