Fix tests by checking block state invariants in the same state context

[abizjak]

Purpose

Fix tests.

Checking state invariants, it is crucial that the invariant checking function is executed in the same context as the state is constructed, since, for example, LMDB database must be available for looking up accounts.

Other uses of assertBlockStateInvariants are correct already.

It's unclear why this test ever worked but possibly it sometimes worked due to unsafePerformIO usage. It did not work on my machine though, it caused an LMDB error.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.