Revert "Revert "Allow to find slapos.libnetworkcache during test.""

For now this is the only working way to test buildout with
slapos.libnetworkcache.

slapos.libnetworkcache development shall be NOT zc.buildout driven.

This reverts commit e3345a7a.

src/zc/buildout/testing.py

@@ -269,7 +269,7 @@ def make_buildout(executable=None):
     """Make a buildout that uses this version of zc.buildout."""
     # Create a basic buildout.cfg to avoid a warning from buildout.
     open('buildout.cfg', 'w').write(
-        "[buildout]\nparts =\n"
+        "[buildout]\nparts =\nfind-links = http://pypi.python.org/pypi/slapos.libnetworkcache"
         )
     # Get state of installer defaults so we can reinstate them (instantiating
     # a Buildout will force the Buildout's defaults on the installer).