cex: improve readability of the subsections

Now that the derivation is no longer printed on one line, aligning the
example and the derivation is no longer useful.  It can actually be
harmful, as it makes the overall structure less clear.

* src/derivation.h, src/derivation.c (derivation_print_leaves): Remove
the `prefix` argument.
* src/counterexample.c (print_counterexample): Put the example next to
its label.
* tests/conflicts.at, tests/counterexample.at, tests/diagnostics.at,
* tests/report.at: Adjust.

src/derivation.c

@@ -428,10 +428,9 @@ derivation_print_flat (const derivation *deriv, FILE *out, const char *prefix)
 }
 
 void
-derivation_print_leaves (const derivation *deriv, FILE *out, const char *prefix)
+derivation_print_leaves (const derivation *deriv, FILE *out)
 {
   int counter = 0;
-  fputs (prefix, out);
   derivation_print_flat_impl ((derivation *)deriv, out, true, &counter, "");
   fputc ('\n', out);
 }