test: evp_extra: test signing with legacy app method based keys

This commit adds `test_EVP_PKEY_sign_with_app_method`, a regression
test for the bug fix in commit 1acc3e8c (pull request #22163).

It is analogous to `test_EVP_PKEY_sign`, only with a fake app method
based key. (The EC key test case was omitted, because there is no
`EC_KEY_METHOD_dup` method.)

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22185)

(cherry picked from commit 860e36d0)