build: remove now-useless --{en, dis}able-tests

build: remove now-useless --{en, dis}able-tests

commit d771020 "don't build tests unless make check is run" has made the
 --{en,dis}able-tests switch completely useless.  The differentiation is
now made by running "make check" or not doing so.  The only effect of
the switch is an "empty" excursion of make into the tests/ directory.

(well, and it turns "make check" useless from the main directory if
 --disable-tests is given, which I don't think makes sense either)

Acked-by: Greg Troxel <gdt@ir.bbn.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>