bitcoin: Overview of slow unit tests

At last IRC meeting we’ve discussed trying to speed up the unit tests, as they’ve become much too slow. For this purpose I’ve run test_bitcoin --log_level=test_suite and picked out the slowest from the list.

The top 20 is (updated 2018-04-14 by @lucash-dev):

Test	Time (μs)
test_big_witness_transaction	39133014
test_CheckQueue_Correct_Random	14808019
knapsack_solver_test	9304171
checkinputs_test	6370348
CreateNewBlock_validity	6105558
test_CheckQueue_Memory	4230330
versionbits_test	3835090
coins_cache_simulation_test	3205869
updatecoins_simulation_test	2499840
merkle_test	2391688
rescan	1900315
SelectCoins_test	1731901
passphrase	1730167
ListCoins	1689793
test_CheckQueue_Catches_Failure	1563869
test_CheckQueue_UniqueCheck	1521864
PrevectorTestInt	1409636
test_CheckQueue_Correct_Max	1401687
bnb_search_test	1147974
cuckoocache_generations	978637

I think (open for discussion, of course) it is unreasonable for a unit test case to take more than ~1s (mind you, we have 242 of them, so each test is supposed to be quick) so we should take a look at each of these up to merkle_test.

It could be as simple in some cases as getting rid of (reasonably heavy) BOOST_* tests in inner loops, moving the checks up to a higher level.

Edit: I added the script to create this table to maintainer-tools: https://github.com/bitcoin-core/bitcoin-maintainer-tools/blob/master/unittest-statistics.py