defmethod: make the function known at compile time.

(defmethod gf ())
(defun f () (gf))
Produced a warning about an undefined function, even though it would
be implicitly created by defmethod.
Fixes lp#503095.
tests: better reports when /bin/ed is not present.

When /bin/ed is not present the test failed with a confusing "unhandled
error".
* Wrap the whole run-program-ed test into with-test, renaming it to run-program-/bin/ed.
* Add before it a test named :is-/bin/ed-installed?.

Making the report more self-describing:
 Failure:            run-program.impure.lisp / IS-/BIN/ED-INSTALLED?
 Failure:            run-program.impure.lisp / RUN-PROGRAM-/BIN/ED
Add --help to run-tests.sh.