Remove use of oteltest in otelrestful

[MrAlias]

• Audit where the oteltest package is used.
• Replace the use of the oteltest package TracerProvider with the trace.NewNoopTracerProvider where no span recording is necessary.
• Move all remaining uses of the oteltest package for testing into a new test module.
  • Create a go.mod with a minimum Go version of 1.15.
  • Add a replace directive for any local contrib packages used in testing.
  • Add a doc.go with package documentation:

    /*
    Package test validates the <PACKAGE_NAME> instrumentation with the default SDK.
    This package is in a separate module from the instrumentation it tests to
    isolate the dependency of the default SDK and not impose this as a transitive
    dependency for users.
    */

• Replace the use of the oteltest package SpanRecorder with the "go.opentelemetry.io/otel/sdk/trace/tracetest".SpanRecorder.
• Refactor any remaining uses of the oteltest package away.