tests: Fix test_xattr

This test was sometimes failing because xattrs set by file link are placed
mainly at the first place, but not always. This commit makes the test to be
invulnerable to this.
Tested characters in xattr names were extended with a new line and a horizontal
tab.

Change-Id: I198eba540fad9f06c1ff5a7d7f7ff74e9a20c244