Merge pull request #2754 from jcanizales/support-test-certificates

src/objective-c/GRPCClient/GRPCCall+Tests.h

src/objective-c/GRPCClient/GRPCCall+Tests.m

src/objective-c/GRPCClient/GRPCCall.m

src/objective-c/GRPCClient/private/GRPCChannel.h

src/objective-c/GRPCClient/private/GRPCChannel.m

src/objective-c/GRPCClient/private/GRPCCompletionQueue.m

src/objective-c/GRPCClient/private/GRPCHost.h

src/objective-c/GRPCClient/private/GRPCHost.m

src/objective-c/GRPCClient/private/GRPCSecureChannel.h

src/objective-c/GRPCClient/private/GRPCSecureChannel.m

src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.h

src/objective-c/GRPCClient/private/GRPCUnsecuredChannel.m

src/objective-c/GRPCClient/private/GRPCWrappedCall.h

src/objective-c/GRPCClient/private/GRPCWrappedCall.m