tests/ass-test-helpers: Improve test performance
tests/ass-test-helpers: Improve test performance

Part of #156. Timing for `./go test assertion-test-helpers` under Bash
3.2.57(1)-release on my MacBook Pro with a 2.9GHz Intel Core i5 CPU and
8GB 1867MHz DDR3 RAM went from the following for the previous commit:

  18 tests, 0 failures

  real    0m6.787s
  user    0m3.259s
  sys     0m3.104s

to:

  18 tests, 0 failures

  real    0m5.907s
  user    0m2.912s
  sys     0m2.603s

mbland/go-script-bash