Fix test_cache_hard_links when on tmpfs

When running with the test root on tmpfs the test_cache_hard_links
fails because in this case du reports 0 as usage for the link instead
of 1 block(s). Adapt the test for that.

Also honor the ZUUL_TEST_ROOT variable if it is present.

Change-Id: Ie5789299de4b23161d80e5375f8a94be109b67f4
1 file changed