split-scripted-test

Split a long scripted test (unittest/scripts/auto/*/scripts/*_norecord.py) into N parallel chunk groups for run_unit_tests.py, or rebalance the groups of a test that is already split, so every group takes about the same time. Use when asked to split, shard, parallelize, balance or rebalance a scripted test, change its number of groups, or find out why one group of a split test is slower than the others.