Change location of current symlink created by test-runner

test-runner should be creating the current symlink in the
directory above the output directory. In a previous commit,
the current symlink was placed in the current working
directory, which could be inaccessible. It is more likely
that the output directory is always accessible.

This is needed because without this there's no deterministic
way to get the path to ZFS Test Suite results until after the
test suite has started. This makes it difficult for buildbot to
follow the log file.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5314