s3:selftest: run test_smbclient_tarmode.pl with a fixed subdirectory name

s3:selftest: run test_smbclient_tarmode.pl with a fixed subdirectory name

$PREFIX is the the value from --with-selftest-prefix.

The result of the test should not depend on --with-selftest-prefix,
the 'long_path' test in particular.

If the path is to long smbclient (via libarchive) will only
put the full path into a PAX HEADER as 'path' keyword,
that's fine in general, modern tools handle it just fine.
But Perl's Archive::Tar don't handle it and only seems
truncated file names.

I have a fix for Archive::Tar, see:
https://git.samba.org/?p=metze/samba/wip.git;a=shortlog;h=c75037d0a06a96cdaca3f3b20a6d237e768b075b

But finishing that is a task for another day, for now I just want to remove
the dependency to --with-selftest-prefix.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>