Use only upstream test code.

At the moment we maintain separate test code for Envoy for OpenSSL primarily due to in-process Envoy server used in each test fixture. 

Investigate allowing for out-of-process Envoy server in the upstream test suite.

Some prior art with respect to running Envoy's integration tests out of process: https://github.com/envoyproxy/nighthawk/blob/07832043239a5a875560ecebc189b75de872065f/test/client_test.cc#L43

It's old, but this used to work back then (and we did this for entirely different reasons back then)So basically this tests forks() and splits code execution for the test fixture and the actual test 

It might serve as as an example if we want to invest into running vanilla upstream's integration test server as a backend to our maistra ssl upstream implementation