Make optional the auto-erase of the records, added by `haveInDatabase()`

首先定位 haveInDatabase() 的实现以及 skip_cleanup_if_failed 的配置处理逻辑。跟踪测试后记录自动删除的位置，然后确认所选的布尔设置可以禁用该行为，同时保留现有的默认设置和失败时的清理行为。

Problem

Currently, all records, added by haveInDatabase() method during test, are automatically removed after the end of the test.
Although this is a documented behaviour, for me it adds more confusion than profit.

In case when I need all the changes, made during test, to disappear after the test, I can't rely solely on this behaviour, because changes may be done not only by haveInDatabase(). So I still need to use other things for keeping tests isolated (wrapping tests in transactions, or to re-populate database from dump before each test, etc).

And in other case, when I do not need tests isolation, I am ok with the fact that each test changes something in database, so auto-erasing some records only adds confusion for me.

So, I'd like to be able to turn this behaviour off in my tests.

Currently we have a skip_cleanup_if_failed configuration parameter, but it does not give full control over the auto-erase, because allows to omit it only when tests fail (which should be the default behaviour, IMHO). Also, its name is confusing (see issue #67).

Proposed solution

Let's add another boolean configuration parameter, responsible for this auto-erase. For example, auto_erase, or cleanup (could be used after renaming current cleanup to repopulate - see issue #67)