eosio_system_powerup_tests/rent_tests is flaky

[ericpassmore]

Transferred from AntelopeIO/reference-contracts#104 original author linh2931

AntelopeIO/reference-contracts#101 fixes under Savanna; it works in any initial block.
eosio_system_powerup_tests/rent_tests works only when head block time is on the second. This requires further investigation.