testing/log: Use lib/bats/helper-function
testing/log: Use lib/bats/helper-function

Applying the `lib/bats/helper-function` functions throughout makes the
log and log assertion tests substantially faster. On my MacBook Pro
before the change:

  ./go test testing/log/assertions log

  92 tests, 0 failures, 2 skipped

  real    1m22.348s
  user    0m36.324s
  sys     0m42.763s

and after the change:

  92 tests, 0 failures, 2 skipped

  real    0m34.204s
  user    0m17.075s
  sys     0m15.461s

mbland/go-script-bash