Fix more tests

* sly-tests.el (sly-test-check-repl-forms): Print failing MREPL
prompt instead of just reporting failure.

* contrib/Makefile: Mimic root Makefile.

* contrib/sly-c-p-c.el (sly-complete-form): Fix failing tests (basic
missing parent insertion.)

* contrib/swank-mrepl.lisp (open-dedicated-output-stream): CMUCL-friendly
external formats.