ZTS: Print warning if running ZTS user_run test locally

Print a warning if you're attempting to run a ZTS test that calls
'user_run', and the ephemeral user doesn't have permissions to
access the test binaries.

This can happen if you're running ZTS from a local git repo. In
that case the test user (say, 'testuser1') may need access to the
ZTS binaries in:

/home/<your_username>/zfs/tests/zfs-tests/bin/

... but 'testuser1' doesn't have permission to enter your home dir:

/home/<your_username>

The warning will help alert users to what is going on. This will
not be an issue when ZTS is actually installed on the system
(via 'make install' or from packages).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17721