test: perform check in view(readonly) mode

Signed-off-by: Benjamin Wang <wachao@vmware.com>

internal/btesting/btesting.go

@@ -117,7 +117,7 @@ func (db *DB) MustReopen() {
 
 // MustCheck runs a consistency check on the database and panics if any errors are found.
 func (db *DB) MustCheck() {
-	err := db.Update(func(tx *bolt.Tx) error {
+	err := db.View(func(tx *bolt.Tx) error {
 		// Collect all the errors.
 		var errors []error
 		for err := range tx.Check() {