bistromathic: beware of portability issues with readline

In some cases readline emits a trailing spaces after the last
suggestion, which results in errors such as:

```
-(       -       atan    cos     exp     ln      number  sin     sqrt$
+(       -       atan    cos     exp     ln      number  sin     sqrt    $
```

Reported by Christopher Nielsen <mascguy@github.com>.
<https://trac.macports.org/ticket/59927#comment:48>
<https://trac.macports.org/attachment/ticket/59927/bison-3.7.6-test-10.13.test-suite.log>

* examples/test (run): Add support for -t.
* examples/c/bistromathic/bistromathic.test: Use it.

examples/c/bistromathic/bistromathic.test

@@ -306,10 +306,17 @@ esac
 sed -e 's/\\t/	/g' >input <<EOF
 (1+\t\t
 EOF
-run 0 '> (1+
+# Nuke the possible trailing white spaces in the effective output.
+# This is to cope with some readlines that pad all the suggestions
+# with white spaces (for alignment), including the last one on a line.
+#
+# See for instance <https://trac.macports.org/ticket/59927#comment:48>
+# and its test-suite.log:
+# <https://trac.macports.org/attachment/ticket/59927/bison-3.7.6-test-10.13.test-suite.log>
+run -t 0 '> (1+
 (       -       atan    cos     exp     ln      number  sin     sqrt
 > (1+
-> ''
+>
 err: 1.4: syntax error: expected - or ( or number or function or variable before end of file
 err:     1 | (1+
 err:       |    ^'

examples/test

@@ -91,18 +91,21 @@ skip ()
 }
 
 
-# run [-noerr, -n] EXPECTED-EXIT-STATUS EXPECTED-OUTPUT [PARSER-OPTIONS]
-# ----------------------------------------------------------------------
+# run [-n, -noerr, -t] EXPECTED-EXIT-STATUS EXPECTED-OUTPUT [PARSER-OPTIONS]
+# --------------------------------------------------------------------------
+# -n: no final end-of-line in expected-output
 # -noerr: ignore stderr, otherwise merge it into effective output.
-# -n: not final end-of-line in expected-output
+# -t: nuke the possible trailing white spaces in the effective output.
 run ()
 {
-  noerr=false
   echo=echo
+  noerr=false
+  rstrip=false
   while true; do
     case $1 in
-      (-noerr) noerr=true; shift;;
       (-n)     echo=printf; shift;;
+      (-noerr) noerr=true; shift;;
+      (-t)     rstrip=true; shift;;
       (*)      break;;
     esac
   done
@@ -129,7 +132,11 @@ run ()
 
   # Combine effective output and error streams.
   {
-    cat out_eff
+    if $rstrip; then
+      sed -e 's/ *$//g' out_eff
+    else
+      cat out_eff
+    fi
     if ! $noerr; then
        sed -e 's/^/err: /g' \
            -e 's/Reducing stack by rule .* (line .*):/Reducing stack by rule XX (line XXX):/g' \