Correctly configure split stress options in format.sh

[2022/03/30 09:30:07.453] format.sh: job in /data/mci/880343efd752addff7fb6d4e1d443391/wiredtiger/cmake_build/test/format/RUNDIR.59 failed

[2022/03/30 09:30:07.454]     t: process 79518 running

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_1=0: ignoring unknown configuration keyword

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_2=0: ignoring unknown configuration keyword

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_3=0: ignoring unknown configuration keyword

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_4=0: ignoring unknown configuration keyword

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_5=1: ignoring unknown configuration keyword

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_6=0: ignoring unknown configuration keyword

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_7=1: ignoring unknown configuration keyword

[2022/03/30 09:30:07.454]     t: WARNING: stress_split_8=0: ignoring unknown configuration keyword