time, runtime: don't set timer when = 0

timer when == 0, in the context of timer0When and timerModifiedEarliest,
is a sentinel value meaning there are no timers on the heap.
TestCheckRuntimeTimerOverflow reaching into the runtime to set a timer
to when = 0 when it is otherwise not possible breaks this invariant.

After golang.org/cl/258303, we will no longer detect and run this timer,
thus blocking any other timers lower on the heap from running. This
manifests as random timers failing to fire in other tests.

The need to set this overflowed timer to when = 0 is gone with the old
timer proc implementation, so we can simply remove it.

Fixes #42424

Change-Id: Iea32100136ad8ec1bedfa77b1e7d9ed868812838
Reviewed-on: https://go-review.googlesource.com/c/go/+/274632
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Pratt <mpratt@google.com>

src/runtime/time.go

@@ -23,6 +23,8 @@ type timer struct {
 	// Timer wakes up at when, and then at when+period, ... (period > 0 only)
 	// each time calling f(arg, now) in the timer goroutine, so f must be
 	// a well-behaved function and not block.
+	//
+	// when must be positive on an active timer.
 	when   int64
 	period int64
 	f      func(interface{}, uintptr)

src/time/internal_test.go

@@ -53,18 +53,8 @@ func CheckRuntimeTimerOverflow() {
 	t := NewTimer(1)
 
 	defer func() {
-		// Subsequent tests won't work correctly if we don't stop the
-		// overflow timer and kick the timer proc back into service.
-		//
-		// The timer proc is now sleeping and can only be awoken by
-		// adding a timer to the *beginning* of the heap. We can't
-		// wake it up by calling NewTimer since other tests may have
-		// left timers running that should have expired before ours.
-		// Instead we zero the overflow timer duration and start it
-		// once more.
 		stopTimer(r)
 		t.Stop()
-		resetTimer(r, 0)
 	}()
 
 	// If the test fails, we will hang here until the timeout in the