Make tests defensive against timezones

My computer uses Asia/Tokyo (+0900) timezone.
Some tests fail under Asia/Tokyo timezone.

```
test_commit(TestObject) [tests/units/test_object.rb:26]:
<"11-08-07"> expected but was
<"11-09-07">.
```

```
test_commit(TestBare) [tests/units/test_bare.rb:21]:
<"11-08-07"> expected but was
<"11-09-07">.
```