otelgrpc: Stablize TestInterceptors

[pellared]

Fixes #1352

The problem is that StreamClientInterceptor ends the spans asynchronously:

opentelemetry-go-contrib/instrumentation/google.golang.org/grpc/otelgrpc/interceptor.go

Lines 305 to 319 in b44dfc9

	stream := wrapClientStream(ctx, s, desc, cfg)
	go func() {
	err := <-stream.finished
	if err != nil {
	s, _ := status.FromError(err)
	span.SetStatus(codes.Error, s.Message())
	span.SetAttributes(statusCodeAttr(s.Code()))
	} else {
	span.SetAttributes(statusCodeAttr(grpc_codes.OK))
	}
	span.End()
	}()

It may seem that it would be better to reimplement StreamClientInterceptor. However, I do not find it pragmatic given that we want to deprecate the interceptors anyway.

EDIT: I think I have implemented a fix for the StreamClientInterceptor here: #4537.