cex: fixes, and enable tests

* 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.

tests/local.mk

@@ -47,6 +47,7 @@ TESTSUITE_AT =                                \
   %D%/c++.at                                  \
   %D%/calc.at                                 \
   %D%/conflicts.at                            \
+  %D%/counterexample.at                       \
   %D%/cxx-type.at                             \
   %D%/diagnostics.at                          \
   %D%/existing.at                             \