runtime: Better test tracebackability of jmpdefer when running a nil defer.

R=bradfitz, dvyukov
CC=golang-dev
https://golang.org/cl/12536046

test/fixedbugs/issue6055.go

@@ -6,11 +6,17 @@
 
 package main
 
+import "runtime"
+
 type Closer interface {
 	Close()
 }
 
 func nilInterfaceDeferCall() {
+	defer func() {
+		// make sure a traceback happens with jmpdefer on the stack
+		runtime.GC()
+	}()
 	var x Closer
 	defer x.Close()
 }