[dmd-internals] dmd tester

[Brad Roberts]

For a while I got a lot of grief for the dmd test suite defaulting to looking for the dmd binary in ../src/.  It's been
overridable for a long time with: make DMD=/path/to/dmd.  But just make DMD=dmd hasn't worked ever (unless the dmd
binary was in the test directory, which I doubt anyone does).

I've been meaning to fix the DMD=dmd case for a long time.  While doing that, if there's a sufficiently large population
that would rather make DMD=dmd be the default behavior rather than ../src, I'll go ahead and do that.  I'll change my
habits and scripts to cope.

Thoughts?

Thanks,
Brad