tests/admin-test.sh: this is a bash script, not a POSIX sh script

The "function foo()" syntax is bash-specific, and Colin indicated in
PR #226 that he prefers to require bash rather than trying to support
every POSIX shell.

Signed-off-by: Simon McVittie <smcv@debian.org>

Closes: #232
Approved by: cgwalters

tests/admin-test.sh

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