tests/bgpd: don't hardcode error number (fix f57000c)

tests/bgpd: don't hardcode error number (fix f57000c)

f57000c ("bgpd: don't send NOTIFY twice for malformed attrs") introduces
BGP_ATTR_PARSE_ERROR_NOTIFYPLS as additional error code that implies the
caller should sent a NOTIFY and convert it to BGP_ATTR_PARSE_ERROR.
Sadly, the latter was hardcoded in bgp_mp_attr_test.c, which now didn't
consider the new value to be an error.

Make the testcase treat all nonzero values as error without discern.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>