Prs/fix unittests active transactions.inactive votes cache election start

[gr0vity-dev]

This testcase has 2 different issues.
This is not meant to be a real fix, but a help for further investigation.

Issue 1

[----------] 1 test from active_transactions
[ RUN      ] active_transactions.inactive_votes_cache_election_start
/Users/bl/Dev/Git/nano-node/nano/core_test/active_transactions.cpp:529: Failure
Expected equality of these values:
  0
  node.active.size ()
    Which is: 1
[  FAILED  ] active_transactions.inactive_votes_cache_election_start (5195 ms)

The reason for this failure is because there is a race condition between the block_cemented_callback and the priority scheduler loop. Sometimes the block gets cemented before the priority scheduler loop has started as you can see in the stats log below:

--> I didn't know how to fix it, but added a delay in the block_cemented_callback to show that the error is gone.

Issue 2

[==========] Running 1 test from 1 test suite.
[ RUN      ] active_transactions.inactive_votes_cache_election_start
nano-node/nano/core_test/active_transactions.cpp:534: Failure
Value of: node.active.empty ()
  Actual: false
Expected: true
[  FAILED  ] active_transactions.inactive_votes_cache_election_start (327 ms)

I relaxed the conditions to timely, so the following will apply :

• when node.ledger.cache.cemented_count is incremented, nano::test::confirmed eventually returns true
• when nano::test::confirmed returns true for all blocks, node.active.empty ()will eventually return true