Commit 870dba07 (Mar 2008) added builddir/src to PATH to pick
up virsh.  Later, virsh was moved to tools; commit db68d6b1
(Oct 2009) noticed this, but only added the new location rather
than deleting the old location.

* tests/Makefile.am (path_add): Drop now-useless directory.
Suggested by Daniel P. Berrange.