Add basic test for docker-ensure-installed.sh script

(intentionally adding this before I fix it so we can see the CI failure light up red and then turn green)

Author: tianon

.test/config.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+# https://github.com/docker-library/official-images/blob/7629e0d0a836d0ec718a6e053753479227cf6825/test/config.sh
+
+imageTests[wordpress]+='
+	wordpress-ensure-installed
+'

.test/tests/wordpress-ensure-installed/run.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+docker run --rm "$1" sh -euc '
+	[ ! -s wp-includes/version.php ] || exit 1
+	docker-ensure-installed.sh
+	[ -s wp-includes/version.php ] || exit 1
+'