tests: use perl for printing special sequences to files.

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 character literals): Use it here for
omitting final newlines.
(Bad escapes in literals): Use it here for special characters.
(cherry picked from commit b70c7fb4e1db54e78d4f3d4a0f110a81118ffc60)