`eosio_system_tests/producer_onblock_check` fails under default `Savanna` libtester

tests/eosio.system_tests.cpp(2943): fatal error: in "eosio_system_tests/producer_onblock_check": critical check true == all_21_produced has failed [true != false]

The test is flaky? If producing 21 more blocks, the test passed:

reference-contracts/tests/eosio.system_tests.cpp

Lines 2889 to 2891 in cd645b5

    
           // give a chance for everyone to produce blocks 
        
           { 
        
              produce_blocks(21 * 12);

Requires a change to the test, see: https://github.com/AntelopeIO/spring/pull/183/files#diff-4de9e7aa369f61849e29127ac678e725621f86011b922101ad418676da630e75R1983

	// give a chance for everyone to produce blocks
	{
	produce_blocks(21 * 12);