x/crypto/internal/wycheproof: implement ECDH tests

[FiloSottile]

They will help test CL 202819.

[rolandshoemaker]

Is the intention here to test the crypto/tls ECDH implementation, and by proxy of needing to decode the test vectors test UnmarshalCompressed/MarshalCompressed?

[gopherbot]

Change https://golang.org/cl/247526 mentions this issue: x/crypto/internal/wycheproof: implement ECDH tests

[gopherbot]

Change https://go.dev/cl/396174 mentions this issue: internal/wycheproof: add ECDH tests