Add initial handshake reassembly tests.

For now, only test reorderings when we always or never fragment messages.
There's a third untested case: when full messages and fragments are mixed. That
will be tested later after making it actually work.

Change-Id: Ic4efb3f5e87b1319baf2d4af31eafa40f6a50fa6
Reviewed-on: https://boringssl-review.googlesource.com/3216
Reviewed-by: Adam Langley <agl@google.com>