Speed up SignatureTest by caching KeyPairs by key algorithm.

This fixes what appears to have been a bug in SignatureTest KeyPair
caching logic. The intended behavior (commit
57f2cc03ff2cf5d2f6413c5410680b4908d7301d) appears to have been that
key pairs are cached by key algorithm, but the implementation didn't
do that. Instead, keys were never cached.

With this change, the number of key pairs generated during
SignatureTest drop from 33 to 3, speeding these tests up.

Change-Id: I187e8d368cdf83f36ec666e474ca1339c6d9cc3a
1 file changed