Add a helper script for key file format conversion tests

As tests are done until now, there are a few scripts that look almost,
but not quite the same.  tkey, tx509, tcrl, tpkcs7, treq, tsid and
probably a few more.

recipes/tconversions.pl is a helper script that generalises the
function of each of those, and can then be used in a general manner
from test recipes.

Reviewed-by: Rich Salz <rsalz@openssl.org>