Fix circular dependency in mock_context

[jdmoody]

There seems to be a legitimate circular dependency here that causes problems when I build this library using Snowpack:

1. ./index.ts imports ./mock_tracer/index.ts
2. ./mock_tracer/index.ts imports ./mock_tracer/mock_tracer.ts
3. ./mock_tracer/mock_tracer.ts imports ./mock_tracer/mock_context.ts
4. ./mock_tracer/mock_context.ts imports ./index.ts

I think it should be fixed by simply importing SpanContext directly rather than importing the full ./index.ts.

[coveralls]

Coverage remained the same at 88.889% when pulling 386e6ad5dfd73225a37bb3d520c94de0eac97477 on jdmoody:fix-mock-context-cycle into 07e847fe95e05ffc5a2e40ce3f0215b889807dc9 on opentracing:master.

[jdmoody]

Thanks for the quick review + merge! 🙏

I'd love to get a patch version with this out, if possible. I believe anyone trying to use opentracing with Snowpack will run into this, otherwise.

[yurishkuro]

Released as v0.14.15