Broken Tornado scope manager doesn't cause errors in tests

[condorcet]

I've been making some experiments with supporting Tornado 6 and found that "broken" Tornado scope manager doesn't cause errors in tests.
For example: in this changes condorcet@91ad6c7 I replaced tracer_stack_context with context manager that do noting. Also I removed Tornado specific code from TornadoScopeManager so it just became ThreadLocalScopeManager.
Build with this changes passed successfully https://www.travis-ci.org/condorcet/opentracing-python/builds/513811185
I haven't found tests that modelling situation of switching context in "concurrent coroutines". I think it should be something like this condorcet@30ac1aa