Default to MK_TESTS=no with bmake

Having this on by default with bmake is not a good idea right now
because there is too much potential fallout in 3rd party packages
and ports for others to be able to use this; see ports-mgmt/pkgng
for one such example.

Discussed-with: sjg
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Revert "Don't compile tests with ports"

This reverts commit 52708edd351559364add1f4fe5c633d4cba6d297.