```
This removes the ability to test against OpenSSL, as well as the OpenSSL verification benchmark.
The motivation is that OpenSSL 3 is deprecating part of the API used here (see #869), and I'm not sure it's worth maintaining. We do lose the fact that this is the only test that verifies randomly-generated cases against an independent implementation. On the other hand, there are tons of existing fixed tests now that test all kinds of edge cases already.
```
Backport of [[https://github.com/bitcoin-core/secp256k1/pull/983 | secp256k1#983]].