unit test active_transactions.fifo occasionally fails

[dsiganos]

The unit test occasionally fails because it expects an election to be dropped once and the election is dropped twice.
This might be due to more than one elections starting for the same block due to race conditions.
This is usally reproduced on my laptop by running the test 1000 times and starting a parallel nano-node compilation.

[ RUN ] active_transactions.fifo
D:\a\nano-node\nano-node\nano\core_test\active_transactions.cpp(1413): error: Expected equality of these values:
1
node.stats.count (nano::stat::type::active_dropped, nano::stat::detail::normal)
Which is: 2
[ FAILED ] active_transactions.fifo (579 ms)

We could fix this by expecting the counter to be any number larger than one but maybe we should also eliminate those duplicate election starts...