xfstests: Convert all tests to use /bin/bash

While most tests use /bin/sh, they are dependent on /bin/sh being a
bash shell.  Convert all the tests to execute via /bin/bash as it is
much, much simpler than trying to debug and remove all the bashisms
throughout the test code.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>

common.dbench

@@ -1,4 +1,4 @@
-##/bin/sh
+##/bin/bash
 #
 # Copyright (c) 2002-2005 Silicon Graphics, Inc.  All Rights Reserved.
 #