tests: fix rename subdir check that was failing in travis

Apparently some or all travis build servers has older than v8.13
coreutils.  The 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
realpath issue.

Reference: https://travis-ci.org/karelzak/util-linux/builds/30538376
Signed-off-by: Sami Kerola <kerolasa@iki.fi>