Deflake third_party/gvisor/pkg/gate/gate_test

TestConcurrentAll executes 1000 goroutines which never sleep,
so they are not preempted by Go's runtime. In Go 1.14, async preemption
has been added, but the added runtime.Gosched() call will do nothing
wrong in this case too.

PiperOrigin-RevId: 301705712

pkg/gate/gate_test.go

@@ -15,6 +15,7 @@
 package gate_test
 
 import (
+	"runtime"
 	"testing"
 	"time"
 
@@ -165,6 +166,8 @@ func worker(g *gate.Gate, done *sync.WaitGroup) {
 		if !g.Enter() {
 			break
 		}
+		// Golang before v1.14 doesn't preempt busyloops.
+		runtime.Gosched()
 		g.Leave()
 	}
 	done.Done()