graphcycles_test: use batch mode in benchmarks
graphcycles_test: use batch mode in benchmarks

So that results are easier to interpret and compare.

Before:
----------------------------------------------------------------
Benchmark                      Time             CPU   Iterations
----------------------------------------------------------------
BM_StressTest/2048        989074 ns       988955 ns          707
BM_StressTest/4096       2043829 ns      2043653 ns          346
BM_StressTest/32768     17073324 ns     17071023 ns           40
BM_StressTest/262144   152278792 ns    152251000 ns            4
BM_StressTest/1048576 1403662723 ns   1397561773 ns            1
BM_ContractEdge/1000      122874 ns       122842 ns         5702
BM_ContractEdge/10000    1471461 ns      1471153 ns          477

After:
----------------------------------------------------------------
Benchmark                      Time             CPU   Iterations
----------------------------------------------------------------
BM_StressTest/2048           482 ns          482 ns      1394688
BM_StressTest/4096           494 ns          494 ns      1409024
BM_StressTest/32768          546 ns          546 ns      1409024
BM_StressTest/262144         568 ns          568 ns      1310720
BM_StressTest/1048576       1176 ns         1176 ns      1048576
BM_ContractEdge/1000         130 ns          130 ns      5271000
BM_ContractEdge/10000        150 ns          150 ns      4680000
