Enabling MK_TESTS by default

[Julio Merino]

Hello all,

I think we are ready to enter the new world so I would like to take
the final step and make MK_TESTS the default.

But before that, I would like to commit
https://phabric.freebsd.org/D174 . I think the change is safe for
current (even though there is the unfortunate need of an entry in
UPDATING). The impact on stable/10 will be evaluated later along with
the move of the libraries to private (which I thought I had done but
hadn't). Feel free to rubber-stamp that diff.

So, unless I hear otherwise:

- Tomorrow I will submit D174 referenced above.
- Flip MK_TESTS locally and run make tinderbox ad nauseam to ensure things work.
- On Sunday or thereabouts commit the MK_TESTS setting to yes by default.

Cheers