The new binary, called |handshaker|, allows split-handshakes to be
tested using shim and handshaker binaries built at different
revisions.
The shim now proxies traffic to the handshaker during the split
handshake. The handoff and handback steps serialize additional state
about the test being performed, and its results.
The proxy and handshaker make heavy use of Unix-isms, and so
split-handshake tests are now restricted to Linux.
Change-Id: I048f0540c3978a31b3e573e00da17caf41a8059e
Reviewed-on: https://boringssl-review.googlesource.com/29348
Reviewed-by: Adam Langley <agl@google.com>
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