runtests.pl: add -L parameter to require additional perl libraries

runtests.pl: add -L parameter to require additional perl libraries

This is useful to change the behaviour of the script without
having to modify the file itself, for example to use a custom
compareparts() function that ignores header differences that
are expected to occur when an external proxy is being used.

Such differences are proxy-specific and thus the modifications
should be maintained together with the proxy.