tests: use perl for printing special sequences to files.

And skip tests if perl is not available.  This is better than
playing tricks with shell portability.  Suggested by Akim
Demaille.
* tests/input.at (Bad escapes in literals): Use it here for special
characters.
(cherry picked from commit b70c7fb4e1)

Conflicts:

	tests/input.at

tests/input.at

@@ -953,7 +953,13 @@ start: '\777' '\0' '\xfff' '\x0'
        '\uffff' '\u0000' '\Uffffffff' '\U00000000'
        '\ ' '\A';
 ]])
-echo 'start: "\T\F\0\1" ;' | tr 'TF01' '\011\014\0\1' >> input.y
+
+# It is not easy to create special characters, we cannot even trust tr.
+# Beside we cannot even expect "echo '\0'" to output two characters
+# (well three with \n): at least Bash 3.2 converts the two-character
+# sequence "\0" into a single NUL character.
+AT_CHECK([[perl -e 'print "start: \"\\\t\\\f\\\0\\\1\" ;";' >> input.y \
+           || exit 77]])
 
 AT_BISON_CHECK([input.y], [1], [],
 [[input.y:2.9-12: invalid number after \-escape: 777