Move test keys into tests

[djc]

IMO this is a more appropriate/conventional home for test data (compared to inside src).

(Context: looking at rustls/rustls#1462 again.)

[ctz]

I feel like behaviour tests that strictly use the public API should also not be in src/ (rationale: it's easy to accidentally neglect to export a public API)

[cpu]

I feel like behaviour tests that strictly use the public API should also not be in src/

Agreed. Moving some of the existing tests seems like a good follow-up.

[djc]

I'll follow up with those changes.