Explicitly disable the build of tests when building bmake.

During "make buildworld", building bmake is (one of) the very first steps
and we should not be building any of its tests.  Conceptually, this is the
right thing to do 1) for build simplicity reasons and 2) because there is
no need to build any tests this early on.

In practice, this fixes tinderbox builds of CURRENT from 9.x when MK_TESTS
is enabled.  This is because bsd.test.mk needs some modern bmake features
not present in 9.x (:tW) and tinderbox is forcing the build to use the
CURRENT share/mk files from the very beginning (see r266617).  By skipping
the build of the tests when still using the host make, we omit the problem.
Arguably, what tinderbox is doing is wrong and needs to be addressed, but
that is a separate issue.