BUILD: Makefile: exclude broken tests by default

BUILD: Makefile: exclude broken tests by default

it could be sometimes a bit confusing to have tests which are known to
be broken executed in the default `make reg-tests` command, especially
for not frequent contributors which are not necessarily aware of all our
quirks.

without this patch, this test is failing on my side:
  #    top  TEST reg-tests/seamless-reload/abns_socket.vtc FAILED (2.228) exit=2
  1 tests failed, 0 tests skipped, 107 tests passed

Signed-off-by: William Dauchy <wdauchy@gmail.com>