fix core_test.go and a race in setting/using mtu

src/core/api.go

@@ -243,16 +243,20 @@ func (c *Core) MaxMTU() uint64 {
 	return c.store.maxSessionMTU()
 }
 
-// SetMTU can only safely be called after Init and before Start.
 func (c *Core) SetMTU(mtu uint64) {
 	if mtu < 1280 {
 		mtu = 1280
 	}
+	c.store.mutex.Lock()
 	c.store.mtu = mtu
+	c.store.mutex.Unlock()
 }
 
 func (c *Core) MTU() uint64 {
-	return c.store.mtu
+	c.store.mutex.Lock()
+	mtu := c.store.mtu
+	c.store.mutex.Unlock()
+	return mtu
 }
 
 // Implement io.ReadWriteCloser