Provide conformance tests for tracers (#191)

* 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

api/testharness/package.go

@@ -0,0 +1,15 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package testharness // import "go.opentelemetry.io/api/testharness"