Remove KYUAFILE=auto as it's the default in bsd.test.mk

Requested-by: jmmv
Move all Makefile magic into Makefile.inc
Add test auto-discovery via functional_test

This simplifies Makefile.inc greatly [1].

Also, toss valid_osreldate support [2]. It was something that made
sense when dealing with backporting tests at EMC, but doesn't make
sense in mainline FreeBSD.

Requested-by: jmmv [1]
Requested-by: jilles [2]
Apply review comments from jmmv

regress -> format_string_functional_test

Use $() instead of ``

Make arguments to check() function more descriptive. Localize them to
the function.
Pick up KYUAFILE from .../tests