eosio_system_tests/producer_wtmsig and eosio_system_tests/producer_wtmsig_transition fail under default Savanna libtester

[linh2931]

1. tests/eosio.system_tests.cpp(847): fatal error: in "eosio_system_tests/producer_wtmsig": critical check control->active_producers().version == 1u has failed [0 != 1]
2. tests/eosio.system_tests.cpp(937): fatal error: in "eosio_system_tests/producer_wtmsig_transition": critical check control->active_producers().version == 1u has failed [0 != 1]

Those two failures are caused by the new way to process proposer schedule in Savanna. 2 rounds of blocks (24) should be produced, instead of 2:

reference-contracts/tests/eosio.system_tests.cpp

Line 846 in cd645b5

produce_blocks(2);