tests/thirdparty: add standard library crypto/ed25519

[mmcloughlin]

Add the standard library crypto/ed25519 package to the third-party test suite.

This is clearly going to be a bit of an edge case as the first attempt #225 shows.

• Building the target Go version with make.bash is slow
• Can we run the tests vs the installed Go version, not the checked out Go version?
• Can we install from a release version, directly or with golang.org/x/dl?
• If we install a release version, how would we handle the case where we test with the -latest flag.
• Can we make use of caching in github actions?

[mmcloughlin]

While I figure out what to do about the standard library test case, I've added #227 to test the package it's vendored from.

cc @FiloSottile