lib: test-data-stack - add some fatal tests.

lib: test-data-stack - add some fatal tests.
Extra caution is necessary as data-stack is such a fundamental component.
All of the brokenness that we add must be undone as soon as possible, or
there will be an endless loop of catastrophic errors. In order to avoid
that, at least try to detect some issues, and abort as quickly as possible.

Alas, due to the reliance of these tests on DEBUG code, if that's not set,
this test is a no-op.

Signed-off-by: Phil Carmody <phil@dovecot.fi>