tests/complete: Extract add_trailing_slashes

Part of #156. Very minor O(0.05s) benefit, but every little bit counts,
especially on Windows.

At this point, the timing for `./go test` 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 merge commit for #161:

  784 tests, 0 failures, 2 skipped

  real    4m16.466s
  user    2m1.907s
  sys     2m2.015s

to:

  real    3m2.408s
  user    1m31.395s
  sys     1m21.114s

Author: mbland

tests/complete/compgen.bats

@@ -58,17 +58,23 @@ teardown() {
   assert_success "${expected[@]/%//}"
 }
 
-@test "$SUITE: adds trailing slashes when called with -f" {
-  mkdir -p "${TEST_GO_ROOTDIR}"/{foo,bar,baz}
-  run "$TEST_GO_SCRIPT" -f -- ''
-
-  # Remember that `compgen` won't add trailing slashes by itself.
-  local expected=($(cd "$TEST_GO_ROOTDIR"; compgen -f -- ''))
+add_trailing_slashes() {
   local i
   for ((i=0; i != "${#expected[@]}"; ++i)); do
     if [[ -d "$TEST_GO_ROOTDIR/${expected[$i]}" ]]; then
       expected[$i]+='/'
     fi
   done
+}
+
+@test "$SUITE: adds trailing slashes when called with -f" {
+  mkdir -p "${TEST_GO_ROOTDIR}"/{foo,bar,baz}
+  run "$TEST_GO_SCRIPT" -f -- ''
+
+  set "$DISABLE_BATS_SHELL_OPTIONS"
+  # Remember that `compgen` won't add trailing slashes by itself.
+  local expected=($(cd "$TEST_GO_ROOTDIR"; compgen -f -- ''))
+  add_trailing_slashes
+  restore_bats_shell_options "$?"
   assert_success "${expected[@]}"
 }