Since the Trace exporter code was ported from https://github.com/census-instrumentation/opencensus-java/tree/master/exporters/trace/stackdriver, the tests could be ported from there as well.

Note that we have dropped the code automatically injecting resource labels based on the environment (i.e. Kubernetes, AWS EC2 etc), so no tests for that functionality needs to be ported.