cmd/cover: fix race in test

Silly test added yesterday requires that some code in a goroutine executes.
Make sure it does.

Change-Id: I7e852454736e300151473986cc437a70b41dc9b7
Reviewed-on: https://go-review.googlesource.com/7691
Reviewed-by: Russ Cox <rsc@golang.org>

cmd/cover/testdata/test.go

@@ -187,9 +187,12 @@ func testEmptySwitches() {
 	switch i := (interface{})(3).(int); i {
 	}
 	check(LINE, 1)
+	c := make(chan int)
 	go func() {
 		check(LINE, 1)
+		c <- 1
 		select {}
 	}()
+	<-c
 	check(LINE, 1)
 }