Make NewTicker panic if d<=0, like time.NewTicker.

This more closely follows the standard library function behavior.

It is directly copied from the standard library: https://cs.opensource.google/go/go/+/refs/tags/go1.20.3:src/time/tick.go;l=23-25

Although panicking is generally bad, it is preferable to mimic the standard library so users see the same behavior between clockwork and the golang standard library.

Without this, some tests can pass with a no op where a real time.Ticker implementation would crash.

clockwork.go

@@ -1,7 +1,9 @@
+// Package clockwork contains a simple fake clock for Go.
 package clockwork
 
 import (
 	"context"
+	"errors"
 	"sort"
 	"sync"
 	"time"
@@ -141,7 +143,14 @@ func (fc *fakeClock) Since(t time.Time) time.Duration {
 
 // NewTicker returns a Ticker that will expire only after calls to
 // fakeClock.Advance() have moved the clock past the given duration.
+//
+// The duration d must be greater than zero; if not, NewTicker will panic.
 func (fc *fakeClock) NewTicker(d time.Duration) Ticker {
+	// Maintain parity with
+	// https://cs.opensource.google/go/go/+/refs/tags/go1.20.3:src/time/tick.go;l=23-25
+	if d <= 0 {
+		panic(errors.New("non-positive interval for NewTicker"))
+	}
 	var ft *fakeTicker
 	ft = &fakeTicker{
 		firer: newFirer(),
@@ -294,9 +303,10 @@ func (fc *fakeClock) set(e expirer, d time.Duration) {
 // The caller must hold fc.l.
 func (fc *fakeClock) setExpirer(e expirer, d time.Duration) {
 	if d.Nanoseconds() <= 0 {
-		// special case - trigger immediately, never reset.
+		// Special case for timers with duration <= 0: trigger immediately, never
+		// reset.
 		//
-		// TODO: Explain what cases this covers.
+		// Tickers never get here, they panic if d is < 0.
 		e.expire(fc.time)
 		return
 	}
@@ -307,7 +317,7 @@ func (fc *fakeClock) setExpirer(e expirer, d time.Duration) {
 		return fc.waiters[i].expiry().Before(fc.waiters[j].expiry())
 	})
 
-    // Notify blockers of our new waiter.
+	// Notify blockers of our new waiter.
 	var blocked []*blocker
 	count := len(fc.waiters)
 	for _, b := range fc.blockers {