countlog/stats/stats_test.go has imported non-exist package

[xnslong]

The countlog/stats/stats_test.go file has imported a non-exist package

import (
	// ...
	"github.com/v2pro/plz/countlog/core"  // no such package
	// ...
)

This will disturb go mod.

github.com/thrift-iterator/go imports
	github.com/v2pro/wombat/generic imports
	github.com/v2pro/plz/countlog imports
	github.com/v2pro/plz/countlog/stats tested by
	github.com/v2pro/plz/countlog/stats.test imports
	github.com/v2pro/plz/countlog/core: module github.com/v2pro/plz@latest found (v0.0.0-20180227161703-2d49b86ea382), but does not contain package github.com/v2pro/plz/countlog/core