Improve Core tests (#131)

In this PR we add some improvements to the core tests.
We extract before and after snapshots to wrapper functions for easier
usage in tests.

We should use `beforeAfterSnapshotWrapper` in the test suites that may
introduce changes to the chain state, so the outcome of test execution
doesn't affect tests in other suites.

I took a change to improve stake tests to some level, but didn't want to
move any further with refactoring not to make this PR unreviewable.