Replace all test private keys with dynamically-generated keys like //.hierarchy about boulder HOT 3 OPEN

There's also some DER encoded private keys floating around such as the following. Note that public keys haven't been sorted out from this.

$ find -type f -iname '*.der'
./test/test-root.key.der
./test/test-ca.der
./test/test-key-5.der
./test/test-ca.key.der
./test/test-root.der

from boulder.

test-ca.der and test-root.der are certificates; I've added the others to the checklist above.

from boulder.

With #7488 out for review, only three clusters of keys remains:

• test-key-5 and the keys hardcoded into wfe_test.go, which correspond to pubkeys hardcoded into mocks/mocks.go and will therefore be a bit more complicated to remove
• the minica keys and certs for mail-test-srv, redis-tls, and wfe-tls, which can largely just be moved into the dynamically-generated PKI, except that some of them are used by unittests too
• //test/hierarchy, which we use nearly everywhere, and have been discussing replacing with a test-only package that generates such certs in-memory at init() time.

from boulder.