[test] Skip LDAP tests if any of the underlying programs is not found Some programs are in ldap-utils or ldapscripts, which may not be installed despite slapd may. Follow-up on 282880f81311.

Handle virtualenv development mode Looking for VIRTUAL_ENV environment variable as a sign of virtualenv activation. This at least works for developing CubicWeb itself (not sure yet about cubes). And it makes `tox --develop` work as well. Add a test for cwconfig._find_prefix within a virtualenv. In tox.ini, use `develop option`_ accordingly. This makes tox running significantly faster since it avoids building and installing the sdist. Aside, the "develop" install is now performed *after* all test requirements are processed, which means that the cubicweb package should always be the one from source -- instead of a released version pulled by a cube through test dependencies, which may occur in "recreate" step. .. _`develop option`: <http://tox.readthedocs.org/en/latest/config.html#confval-usedevelop=BOOL

[skeleton] Use os.walk in setup.py for Python3 compatibility

[skeleton] Use unittest.main directly Instead of lgc.testlib.unittest_main as they are identical.