tests: fix rename subdir check that was failing in travis

tests: fix rename subdir check that was failing in travis

Apparently some or all travis build servers has a bit older coreutils
version.  The coreutils v8.13 added a feature to readlink that allows
user to specify multiple arguments.  Earlier versions gave exactly the
type of error travis_script log has.

+readlink: extra operand `rename_aa/sublink.2'
+Try `readlink --help' for more information.

The commit 0f85e0535bd4e6d19fadca182e4d57660b6bb57b introduced the travis
readlink issue, and has caused failures for 8 days in row (builds 192 -
213).

Reference: https://travis-ci.org/karelzak/util-linux/builds
Reviewed-by: Bernhard Voelker <mail@bernhard-voelker.de>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>