`tests/opt-testcases/tls13-compat.sh` is supposed to be automatically
generated by `tests/scripts/generate_tls13_compat_tests.py`. So far, the
output has been updated by running the script manually and committing the
output. Switch to using our framework for generated files.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>