create_test_env.sh: use sh instead of bash

this is actually a regular sh-portable script, and this prevents needing
bash at build time or for tests (for packaging from tarballs)

app/tests/create_test_env.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 # Copy tests resources
 for dir in files gimpdir gimpdir-empty; do