Add GRPC + TLS integration tests (notably, test important security properties)

[matthild]

This pull request adds end-to-end tests of the following basic functionality:

• A server configured to require client authentication refuses to accept connections from a client that has an untrusted certificate.
• A server configured to require client authentication actually does require client authentication.
• A client and a server configured using GrpcSslContexts can successfully communicate with each other. (This last test fails -- any idea why?)

These tests work by having an actual GRPC client talk through the operating system's network stack to an actual GRPC server listening on a port, and thus mimic how GRPC will behave in real-world applications.

(I have also drafted tests of some other important security properties, but suggest adding them later via a separate pull request, once these initial tests are in place. The additional tests will be easier to add in a sensible way if some organization of the test certificates directory happens first.)

[grpc-kokoro]

Can one of the admins verify this patch?

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[matthild]

Regarding the request to sign the CLA: I have already registered my GitHub account internally, as a Google employee. Should I sign the CLA also?

[ejona86]

@matthild, the CLA is based on the email address of the commits (i.e., mh@matth-red.local), not your GitHub account.