test: return a dir from treegen.prepare_directory()

It makes the helper a bit more convenient to use it tests.

NO_DOC=testing helper
NO_TEST=see NO_DOC
NO_CHANGELOG=see NO_DOC

(cherry picked from commit f81b1aac)

test/treegen.lua

@@ -58,6 +58,7 @@ function treegen.write_script(dir, script, body)
     local fh = fio.open(script_abspath, flags, mode)
     fh:write(body)
     fh:close()
+    return script_abspath
 end
 
 -- Generate a script that follows a template and write it at the