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Use the same test files as other client cert tests. While I'm here, use
the new BoringSSL scopers. This is done in preparation for to make
our ECDSA client certs logic for TLS 1.3. (Having certificates with the
keys will be convenient)

Also check in client_root_ca.pem so the inputs are all available.

BUG=630150

Review-Url: https://codereview.chromium.org/2390493002
Cr-Commit-Position: refs/heads/master@{#423029}