add common test funcs to internal/test

[judwhite]

Promoting some common test functions in the project to its own internal package.

Example Usage:

import (
    "testing"
    "github.com/nsqio/nsq/internal/test"
)

func TestAwesomesauce(t *testing.T) {
    // do some awesomeness
    test.Equal(t, svc.StartPending, changes[0].State)
    test.Nil(t, wsf.ws.getError())
}

[mreiferson]

LGTM