Add TESTDIR support

This allows me to manually change the directory that test sources will
come from for a given target.  The idea is that this way I can
sepecify that multiple targets point to exactly the same tests, which
is useful for generating a large number of configurations.
Allow executable CONFIGs

I guess I've finally proved the 11th rule of programming (though it's
BASH this time... :)).  That said, it is somewhat useful to generate
really, really large Configfiles sometimes.  In this case I'm
attempting to explore a huge design space for CS250.
Bump version to 0.5.4