* Add inspectable test tracer
This adds a test Tracer implementation that tracks its active and ended
spans and uses a Span implementation that can be inspected (e.g., to see
what attributes have been set).
* Ensure test tracer can start spans concurrently
* Flip conditional logic to return early
* Remove duplicate test
* Fix file name casing
🤦
* Add comments to testtrace code
* Remove Link and AddLink methods from test Span
* Enable concurrently setting and getting test attrs
* Remove SetAttribute from test tracer
* Fix test
* Fix names post-rebase
