Implemented verbose mode.

Implemented verbose mode.
  make test VERBOSE=1
should now produce the same output like
the Makefile normally does.
Without VERBOSE only the names of failed tests
are printed.

I am pretty comfortable with this version, and
I think it is time to switch over to Makefile.parallel
to be the default.

What do you think? Did I forget some piece of feedback?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@48458 91177308-0d34-0410-b5e6-96231b3b80d8