test framework: Mock DispatchHttpCall to return error

[rodrigovc]

I am doing unit tests for my filter with DispatchHttp, something like this:

_, err = proxywasm.DispatchHttpCall(ctx.clusterName, headers, body, nil, ctx.timeout, ctx.callback)
	if err != nil {
		proxywasm.LogCriticalf("big error: %v", err)
		proxywasm.ResumeHttpRequest()
		return types.ActionContinue
	}

I don't know of a way to mock the DispatchHttpCall method in order to return an error so we can cover this part of the code.

Is there any way of doing this?

[mathetake]

#451