test/evp_test: Test if EVP_DigestSign() set signature's length.

Increase the signature's length, that is passed to EVP_DigestSign(). The
implementation should set this parameter back to the actual length, that
has been written.

This (oneshot_digestsign_test_run()) fails for the ed25519/ ed448
implementation of s390 prio the fix. The change in digestsign_test_run()
follows the same pattern and is for collecting bonus points.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/18928)

(cherry picked from commit fc5888cc)