Add make target to build geocouch for unit tests

geocouch needs a special build in order to run unit tests. With the
make target `geocouch-build-for-testing` this build will be created
in a subdirectory called `geocouch-for-testing`. From within this
directory you can then run `make test` to run the unit tests.

Change-Id: I0d932bd80dd31f355e48cfa15d55fc8bfa3f6b95
Reviewed-on: http://review.couchbase.org/41992
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>