If you found a bug in python-ldap, or would request a new feature,
this is the place to let us know.

Please describe the issue and your environment here.

Issue description:
I'm working in packaging python ldap for Clear Lunix but slapd tests fails when starting slapd due to not compiled with SASL support.
This is the line which raises the RuntimeException: https://github.com/python-ldap/python-ldap/blob/master/Lib/slapdtest/_slapdtest.py#L407
The exception message looks like this:

*** RuntimeError: ['/usr/bin/ldapwhoami', '-H', 'ldapi://%2Fhome%2Fgbriones%2Fgithub%2Fpython-ldap%2Fpython-ldap-test-56615%2Fldapi', '-Y', 'EXTERNAL', '-Q'] process failed:
b''
b'ldapwhoami: not compiled with SASL support\n'

Steps to reproduce:

1. Build with not SASL support
2. Run tests

Operating system: Clear Linux 21060

Python version: 3.6.4

python-ldap version: 3.0.0