Use Cleanup to remove old entries in ts map

assert.go

@@ -24,6 +24,10 @@ func New(t T) Assert {
 	f := runtime.FuncForPC(reflect.ValueOf(a).Pointer())
 	ts[f] = t
 
+	t.Cleanup(func() {
+		delete(ts, f)
+	})
+
 	return a
 }
 

type.go

@@ -4,4 +4,5 @@ package assert
 type T interface {
 	Error(...interface{})
 	Helper()
+	Cleanup(func())
 }