examples: beware of portability issues with sh's trap

On AIX 7.2, when invoking "exit 77", we actually exit with 127.  The
"cleanup" function, called via trap, received an incorrect exit
status, something described in Autoconf's doc.
Reported by Bruno Haible.
https://lists.gnu.org/archive/html/bug-bison/2020-05/msg00029.html
https://lists.gnu.org/archive/html/bug-bison/2020-05/msg00047.html

* examples/test (skip): New.
* examples/c/bistromathic/bistromathic.test,
* examples/c/reccalc/reccalc.test: Use it, to ensure $? is set to 77
when the trap is called.

examples/c/bistromathic/bistromathic.test

@@ -46,8 +46,7 @@ if diff perfect effective >/dev/null 2>&1; then
 elif diff ok effective >/dev/null 2>&1; then
     strip_prompt=true
 else
-    echo "SKIP: this is not the GNU Readline we expect"
-    exit 77
+    skip "this is not the GNU Readline we expect"
 fi