Add new env parameter:MULTIREAD_BATCHED for regression_test.sh

[lhsoft]

Fix: #9907

Test Plan: MULTIREAD_BATCHED=false ./tools/regression_test.sh

[mdcallag]

Repeating what was discussed elsewhere, benchmark.sh --multiread_batched=true is another way to accomplish this. Seeing this diff reminded me to update my benchmark scripts to do that, so something good has come from this.