tests/admin-test.sh: add #!/bin/sh

This is executable and appears to be intended to be executed directly,
so it should have a #! line.

tests/admin-test.sh

@@ -1,4 +1,4 @@
-#
+#!/bin/sh
 # Copyright (C) 2011,2014 Colin Walters <walters@verbum.org>
 #
 # This library is free software; you can redistribute it and/or