Replication unit test tweaks

Added the self-signed X.509 cert (used by CBL_Puller_SSL test case) to
the project, and changed the way external test files are loaded to make
it work on iOS too.
Made continuous change tracker able to handle self-signed server certs

Made WebSocket class able to use custom SSL settings and added a delegate
method to report the SecTrustRef. CBLWebSocketChangeTracker then checks
the trust the same way as CBLSocketChangeTracker.
For #444