* Provide conformance tests for tracers
The test harness may be used to ensure that a given tracer behaves
according to the expectations set by the API.
* Add `ToMatchError` matcher
* Use DeepEqual to compare unknown types in matchers
Unlike basic `==`/`!=`, `reflect.DeepEqual` can compare arbitrary
types (e.g., `[]string` to `[]string`)
* Use struct instead of string for test context key
