Previously, this test didn't provide an obvious indication that it
had all succeeded, and assumed that it was run as root with $(pwd) in
/usr/lib/apertis-tests or a git clone of apertis-tests.

To use this as a manual test:

    Prerequisites:
        - apertis-tests installed (sudo apt install apertis-tests)
    Steps:
        sudo /usr/lib/apertis-tests/common/sanity-check
    Successful result:
        Output ends with "# Sanity check successful"
    Failing result:
        Output ends with "Failed, see above for details" or ends with
        neither of the expected messages
    Diagnosing failures:
        Look for "TEST_RESULT:fail:" in the output (it may appear more
        than once), and look for warnings/errors shortly before each
        occurrence of that message

Reviewed-by: Luis Araujo <luis.araujo@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4284