Make mmutex actually acquire and release file locks

mmutex/lib/flock_test.go

-// Copyright 2017 The LUCI Authors. All rights reserved.
-// Use of this source code is governed under the Apache License, Version 2.0
-// that can be found in the LICENSE file.
-
-package lib
-
-import (
-        "io/ioutil"
-        "os"
-        "testing"
-
-        . "github.com/smartystreets/goconvey/convey"
-
-        . "github.com/luci/luci-go/common/testing/assertions"
-)
-
-func TestAcquireExclusiveLock(t *testing.T) {
-        lockDir, err := ioutil.TempDir("", "")
-        if err != nil {
-                t.Error("received error creating temporary testing directory")
-        }
-        defer os.Remove(lockDir)
-        path := lockDir + "test.lock"
-
-        Convey("AcquireExclusiveLock errors if lock file doesn't exist", t, func(c C) {
-                So(AcquireExclusiveLock(path), ShouldErrLike, "lock file does not exist")
-        })
-}