Use /tmp directory for temp file generated with mktemp.

Moving to `mktemp` with files in /tmp makes it possible to remove the
local test/tmp directory.

.gitignore

@@ -1,2 +0,0 @@
-test/tmp/*
-!test/tmp/.keep

test/test_helper.bash

@@ -4,10 +4,15 @@ setup() {
   # The location of the `hosts` script being tested.
   export _HOSTS="${BATS_TEST_DIRNAME}/../hosts"
 
-  export HOSTS_PATH="${BATS_TEST_DIRNAME}/tmp/hosts"
-  cp "${BATS_TEST_DIRNAME}/fixtures/hosts" "${BATS_TEST_DIRNAME}/tmp/hosts"
+  export HOSTS_PATH="$(mktemp /tmp/hosts_test.XXXXXX)" || exit 1
+  cat "${BATS_TEST_DIRNAME}/fixtures/hosts" > "${HOSTS_PATH}"
 }
 
 teardown() {
-  rm "${BATS_TEST_DIRNAME}/tmp/hosts"
+  if [[ -n "${HOSTS_PATH}" ]] &&
+     [[ -e "${HOSTS_PATH}" ]] &&
+     [[ "${HOSTS_PATH}" =~ ^/tmp ]]
+  then
+    rm "${HOSTS_PATH}"
+  fi
 }