De-race the tests; support concurrent Mock::Add; add Mock::Stop support

[jmacd]

I'm happy to contribute several improvements.

I removed the tests for the real clock because it's basically impossible to make them not racey. The test now passes w/ Go-1.10 under the race detector.

[3onyc]

There seems to still be a race in .Reset(), minimal test case:

package main

import (
        "github.com/jmacd/clock"
        "time"
)

func main() {
        cl := clock.NewMock()
        timer := cl.Timer(1 * time.Second)
        go func() {
                <-timer.C
                timer.Reset(2 * time.Second)
        }()
        cl.Add(2 * time.Second)
        <-time.After(5 * time.Second)
}

run with go run -race should fail most of the time

==================
WARNING: DATA RACE
Read at 0x00c4200a6038 by goroutine 5:
  github.com/jmacd/clock.(*Timer).Reset()
      ~/go/src/github.com/jmacd/clock/clock.go:311 +0xe9
  main.main.func1()
      ~/main.go:13 +0x69

Previous write at 0x00c4200a6038 by main goroutine:
  github.com/jmacd/clock.(*Timer).execute()
      ~/go/src/github.com/jmacd/clock/clock.go:330 +0x81
  github.com/jmacd/clock.(*Mock).Set()
      ~/go/src/github.com/jmacd/clock/clock.go:85 +0x49
  github.com/jmacd/clock.(*Mock).Add()
      ~/go/src/github.com/jmacd/clock/clock.go:76 +0x94
  main.main()
      ~/main.go:15 +0x128

Goroutine 5 (running) created at:
  main.main()
      ~/main.go:11 +0x111
==================
Found 1 data race(s)

[jmacd]

Recommend https://github.com/jonboulle/clockwork