cmd/vet: make tests safe for perl 5.26

Someday we should write errchk in Go. But not today.

Fixes #20007

Change-Id: I61ccf3cfaa66a710782f8a2212a6a2b4040698da
Reviewed-on: https://go-review.googlesource.com/40950
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/testdata/copylock_func.go

@@ -87,9 +87,9 @@ func FuncCallInterfaceArg(f func(a int, b interface{})) {
 	f(2, &t)
 	f(3, &sync.Mutex{})
 	f(4, m) // ERROR "call of f copies lock value: sync.Mutex"
-	f(5, t) // ERROR "call of f copies lock value: struct{lock sync.Mutex} contains sync.Mutex"
+	f(5, t) // ERROR "call of f copies lock value: struct.lock sync.Mutex. contains sync.Mutex"
 	var fntab []func(t)
-	fntab[0](t) // ERROR "call of fntab.0. copies lock value: struct{lock sync.Mutex} contains sync.Mutex"
+	fntab[0](t) // ERROR "call of fntab.0. copies lock value: struct.lock sync.Mutex. contains sync.Mutex"
 }
 
 // Returning lock via interface value
@@ -105,7 +105,7 @@ func ReturnViaInterface(x int) (int, interface{}) {
 	case 2:
 		return 2, m // ERROR "return copies lock value: sync.Mutex"
 	default:
-		return 3, t // ERROR "return copies lock value: struct{lock sync.Mutex} contains sync.Mutex"
+		return 3, t // ERROR "return copies lock value: struct.lock sync.Mutex. contains sync.Mutex"
 	}
 }