The test case test_split_content fails on RHEL 9 and Fedora 41 because
their OpenSSL packages do not accept SHA-1 signatures. This was only
caught after commit https://github.com/ruby/openssl/commit/69fd7f886313 added the missing assertion.

While the example PKCS#7 structures could be simply regenerated with
SHA-256, this test case could be simplified because it is checking two
different things.

Replace test_split_content with separate test cases: one verifying
signed-data authenticatedAttributes and another for decoding BER input.

Fixes https://github.com/ruby/openssl/issues/875

https://github.com/ruby/openssl/commit/b32406b0c1