Allow running the compliance test against specified runners

[rakyll]

As someone who is only doing work for the OpenTelemetry collector, it'd be great to be able to run the tests only against Otel. Currently, I comment out the rest of the runners to achieve this behavior easily but it'd be nice if I was able to configure the tests via an environment variable.

[tomwilkie]

Hi Jaana! I stuck something in the readme on how to do this:

$ go test -run "TestRemoteWrite/otelcollector/.+" -v ./

I agree though, I want to make this much easier / flag or env based and ditch running these as unit tests when it makes sense, so I'll leave this open.

[rakyll]

What you are suggesting is pretty reasonable especially with a reference snippet on the README.