testing/synctest: add Sleep

Add a convenience function which combines time.Sleep and synctest.Wait.

Fixes #77169

Change-Id: I2ff105105e95cfd8e5b4f72ccacf7afa59efb6bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/740066
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

api/next/77169.txt

@@ -0,0 +1 @@
+pkg testing/synctest, func Sleep(time.Duration) #77169

doc/next/6-stdlib/99-minor/testing/synctest/77169.md

@@ -0,0 +1 @@
+The new [Sleep] helper function combines [time.Sleep] and [testing/synctest.Wait].

src/testing/synctest/synctest.go

@@ -268,6 +268,7 @@ package synctest
 import (
 	"internal/synctest"
 	"testing"
+	"time"
 	_ "unsafe" // for linkname
 )
 
@@ -309,3 +310,23 @@ func testingSynctestTest(t *testing.T, f func(*testing.T)) bool
 func Wait() {
 	synctest.Wait()
 }
+
+// Sleep blocks until the current bubble's clock has advanced
+// by the duration of d and every goroutine within the current bubble,
+// other than the current goroutine, is durably blocked.
+//
+// This is exactly equivalent to
+//
+//	time.Sleep(d)
+//	synctest.Wait()
+//
+// In tests, this is often preferable to calling only [time.Sleep].
+// If the test itself and another goroutine running the system under test
+// sleeps for the exact same amount of time, it's unpredictable which
+// of the two goroutines will run first. The test itself usually wants
+// to wait for the system under test to "settle" after sleeping.
+// This is what Sleep accomplishes.
+func Sleep(d time.Duration) {
+	time.Sleep(d)
+	Wait()
+}