xfstests: Convert all tests to use /bin/bash

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>