ZTS: Fix incorrect use of libtest in user_run by xattr_003_neg

You can't use user_run to eval ksh functions defined in libtest unless
you include libtest in the user shell.

Fix xattr_003_neg by:

• include libtest in the user shell
• *then* run get_xattr
• assert this fails
• use variables for filenames so they don't change in the user's shell
• don't log the contents of /etc/passwd
• cleanup all byproducts

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11185