tests: Add an option to clean up the testvol file unconditionally
    
    Failed tests skip cleanup, so if the final test doesn't remove the
    testvol then it is left around in the test environment. For CI we want
    to remove it so provide a --always-clean-testvol option which cleans up
    unconditionally in an EXIT trap routine.
    
    This can be used via 'make check TOPTS=--always-clean-testvol'
    
    Signed-off-by: Andrew Price <anprice@redhat.com>