tests/thirdparty: standard library tests are slow

[mmcloughlin]

#225 and #226 added third-party tests for crypto/ed25519. However, this test is slow, largely due to having to run make.bash to compile the Go toolchain.

It would be good to fix this. See #226 for some ideas. Perhaps the best way to do this would be to allow the "checkout" phase of the test to be customizable, allowing this particular test to use a pre-compiled Go distribution rather than a fresh git checkout.

[mmcloughlin]

#355 has the same problem, so now this issue is twice as bad.

If we can't fix the root cause, you could imagine restructuring it so that the two stdlib tests share a common setup step, and the make.bash is run only once.