sync: throw, not panic, for unlock of unlocked mutex

This was originally done in https://golang.org/cl/31359 but partially
undone (apparently unintentionally) in https://golang.org/cl/34310

Fix it, and update tests to ensure the error is unrecoverable.

Fixes #23039

Change-Id: I923ebd613a05e67d8acce77f4a68c64c8574faa6
Reviewed-on: https://go-review.googlesource.com/82656
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/sync/mutex.go

@@ -118,7 +118,7 @@ func (m *Mutex) Lock() {
 			// The goroutine has been woken from sleep,
 			// so we need to reset the flag in either case.
 			if new&mutexWoken == 0 {
-				panic("sync: inconsistent mutex state")
+				throw("sync: inconsistent mutex state")
 			}
 			new &^= mutexWoken
 		}
@@ -140,7 +140,7 @@ func (m *Mutex) Lock() {
 				// inconsistent state: mutexLocked is not set and we are still
 				// accounted as waiter. Fix that.
 				if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
-					panic("sync: inconsistent mutex state")
+					throw("sync: inconsistent mutex state")
 				}
 				delta := int32(mutexLocked - 1<<mutexWaiterShift)
 				if !starving || old>>mutexWaiterShift == 1 {
@@ -181,7 +181,7 @@ func (m *Mutex) Unlock() {
 	// Fast path: drop lock bit.
 	new := atomic.AddInt32(&m.state, -mutexLocked)
 	if (new+mutexLocked)&mutexLocked == 0 {
-		panic("sync: unlock of unlocked mutex")
+		throw("sync: unlock of unlocked mutex")
 	}
 	if new&mutexStarving == 0 {
 		old := new

src/sync/mutex_test.go

@@ -155,7 +155,10 @@ func init() {
 	if len(os.Args) == 3 && os.Args[1] == "TESTMISUSE" {
 		for _, test := range misuseTests {
 			if test.name == os.Args[2] {
-				test.f()
+				func() {
+					defer func() { recover() }()
+					test.f()
+				}()
 				fmt.Printf("test completed\n")
 				os.Exit(0)
 			}