This required making the 'ghc-config-mk' variable overridable in
testsuite/mk/boilerplate.mk, and then making use of this in hadrian
to point to '<build root>/test/ghcconfig' instead, which is where we
always put the test config.
Previously, we would build ghc-config and run it against the
GHC to be tested, a second time, while we're running the tests, because some
include testsuite/mk/boilerplate.mk. This was causing unexpected output