test/recipes/test_genrsa.t : don't fail because of size limit changes

There is a test to check that 'genrsa' doesn't accept absurdly low
number of bits.  Apart from that, this test is designed to check the
working functionality of 'openssl genrsa', so instead of having a hard
coded lower limit on the size key, let's figure out what it is.

Partially fixes #5751

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5754)