Remove span filter for GCP resource detector HTTP calls in tests

[david-luna]

While working #214 the tests revealed that @opentelemetry/instrumentation-http is generating spans for the GCP cloud resource detector. This happens because the detector needs to request som info from specific URLs.

This is an issue to track the the upstream's issue (not created yet). Once solved we must create a PR to remove the filter added in testutils.js file.