https://github.com/ruby/openssl/pull/958 changed the common logic for
digest algorithm lookup:

 - If the argument is neither an OpenSSL::Digest instance nor a String,
   it is now implicitly converted to String with #to_str. This is
   consistent with algorithm name lookup logic in ruby/openssl for pkeys
   and ciphers.

 - If the name is not recognized, OpenSSL::Digest::DigestError is raised
   instead of RuntimeError.

Update the specs accordingly:

 - Remove specs that expect #to_str not to be called.

 - Relax regexps matching TypeError messages.

 - Expect OpenSSL::Digest::DigestError instead of RuntimeError for
   ruby/openssl 4.0.0 and later.

LibreSSL seems not to support `scrypt`.

https://rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20231105T233005Z.fail.html.gz
```
1)
OpenSSL::KDF.scrypt creates the same value with the same input ERROR
NoMethodError: undefined method `scrypt' for module OpenSSL::KDF
```

This reverts commit d434765faead1583ca9008bb579067a288085b93.