Fixes the SEGV in test 247 (counterexample.at:195): "S/R after first
token".
* src/counterexample.c: here.
* tests/counterexample.at: Fix expectations.
* src/counterexample.c, src/derivation.c:
Do not output diagnostics on stdout, that's the job of stderr, and the
testsuite heavily depend on this.
Do not leave trailing spaces in the output.
* tests/counterexample.at: Use AT_KEYWORDS.
Specify the expected outputs.
* tests/local.mk: Add counterexample.at.
