Integrate w3c TraceContext validation into CI and tox

[toumorokoshi]

There is an external validator to ensure that the TraceContext spec is adhered to:

https://github.com/w3c/trace-context/tree/master/test

We should integrate this as a test that is executed for CI. Probably with a version pinned so we don't randomly break CI as new specifications are added.

[reyang]

This should have been addressed by @toumorokoshi in #228.