tests/complete: Source GO_USE_MODULES at top
tests/complete: Source GO_USE_MODULES at top

Part of #156. Timing for `./go test complete` 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:

  10 tests, 0 failures

  real    0m4.536s
  user    0m2.158s
  sys     0m2.162s

to:

  10 tests, 1 failure

  real    0m3.133s
  user    0m1.575s
  sys     0m1.406s

mbland/go-script-bash