ddtracer/tracer: Test children marked as "top level" correctly

[evanj]

This modifies TestTracerStartChildSpan to check that the "top level" metric is set appropriately. This test now verifies the following condition in tracer.StartSpan. Without this change, changing the boolean expression does not cause any tests to break. This is needed because I think there is a race condition here, but I want to make sure there are tests before changing it.

The tested condition is:

if context == nil || context.span == nil || context.span.Service != span.Service {
  span.setMetric(keyTopLevel, 1)
  // all top level spans are measured. So the measured tag is redundant.
  delete(span.Metrics, keyMeasured)
}