Add support for using mock clock in cron

[sarsharma]

This PR uses https://github.com/benbjohnson/clock to mock the time.
Adding support for using mock clock with cron will be helpful to write test cases and verify cron behaviour.

Similar to this PR #327 but uses a more up to date mock clock

PR also contains fix for flaky unit tests on windows and minor linting fixes

[CybotTM]

Solution available in maintained fork

This feature has been implemented in netresearch/go-cron, a maintained fork of this library.

Implementation: Full mock clock support with FakeClock that enables deterministic testing:

// Create a fake clock initialized to a specific time
fakeClock := cron.NewFakeClock(time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC))
c := cron.New(cron.WithClock(fakeClock))

// Add jobs as usual
c.AddFunc("@every 1h", func() {
    fmt.Println("Triggered!")
})

c.Start()

// Wait for scheduler to register timer
fakeClock.BlockUntil(1)

// Advance time deterministically - no real waiting!
fakeClock.Advance(time.Hour) // Triggers the job immediately

// Check timer count for assertions
count := fakeClock.TimerCount()

Features:

• NewFakeClock(t) - Initialize to specific time
• Advance(d) - Move time forward, firing expired timers
• Set(t) - Jump to specific time
• BlockUntil(n) - Wait for n timers to be registered
• TimerCount() - Get active timer count

How to migrate:

// Change import from:
import "github.com/robfig/cron/v3"

// To:
import cron "github.com/netresearch/go-cron"

The API is 100% backward compatible. See the migration guide for details.

Related: netresearch#4, netresearch#34