examples: fix handling of syntax errors

The shell grammar does not allow empty statements in then/else part of
an if, but examples/test failed to catch the syntax errors from the
script it ran.  So exited with success anyway.

You would expect 'set -e' to suffice, but with bash 3.2 actually it
does not.  As a matter of fact, I could find a way to have this behave
properly:

    $ cat test.sh
    set -e
    cleanup ()
    {
      status=$?
      echo "cleanup: $status"
      exit $status
    }
    trap cleanup 0 1 2 13 15
    . $1
    s=$?
    echo "test.sh: $s"
    exit $s

    $ cat bistro.test
    if true; then
    fi

    $ /bin/sh ./test.sh ./bistro.test
    ./bistro.test: line 2: syntax error near unexpected token `fi'
    cleanup: 0
    $ echo $?
    0

Remove the set -e (or the trap), and tada, it works...  So we have to
deal with the error by hand.

* examples/test ($exit): Replace with...
($status): this.
Preserve the exit status of the test case.
* examples/c/bistromathic/bistromathic.test: Fix syntax error.

examples/c/bistromathic/bistromathic.test

@@ -43,6 +43,7 @@ echo '0' | prog >effective
 
 if diff perfect effective >/dev/null 2>&1; then
     # Alles ist gut.
+    strip_prompt=false
 elif diff ok effective >/dev/null 2>&1; then
     strip_prompt=true
 else