According to the documentation, the test module included with the LDAP module should contain the mock LDAP server.
Where can I find it after enabling this module? How should I configure it so that I can start testing logging in with a LDAP server?
The "Mock LDAP" server that is being used for the LDAP Test module is, in fact, a mock server object. It allows the tests to run, and doesn't exist outside the scope of the simpletests.
If you're looking to provide a way to "mock" an LDAP instance on your development site, it can't provide that functionality.
If you're trying to write your own simpletests for LDAP integration, check out the Examples for Developers project. The included module,
simpletest_example, has excellent documentation for writing simple tests.