Fix W3C TraceContext Validation Tests.

[tidal]

trafficstars

At the moment the Tests to tun the W3C Distributed Tracing Validation Service are broken and need to be fixed. Also the tests setup is rather convoluted as it creates a Symfony project to run the tests against. This can be much more simplified by just using a single file as the entry point.

• Use the Base Docker Image in the docker-compose file to speed up the test setup
• Simplify test setup by using a single PHP file.
• Fix the tests and/or TraceContextPropagator

[bobstrecansky]

@yuktea is going to work on this.