topotests: add bgp_asdot_regex test

topotests: add bgp_asdot_regex test

This test ensures that the regex used to filter as paths has to
be expressed in the asnotation of the BGP instance where prefixes
are received. 2 aspaths have been forged, both for AS 65540, but
only the former is expressed in asdot. If the local BGP instance
is expressed in asdot format, then only the former ASPATH will
match properly the incoming update. Reversely, when the local BGP
instance is expressed in plain format, then only the latter ASPATH
will match properly the incoming update.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>