add initial user filter spec

spec/ee/spec/lib/ee/gitlab/ldap/user_filter_spec.rb

+require 'spec_helper'
+
+describe EE::Gitlab::LDAP::UserFilter do
+  include LdapHelpers
+
+  let(:auth_hash) do
+    OmniAuth::AuthHash.new(uid: 'uid=john,ou=people,dc=example,dc=com', provider: 'ldapmain')
+  end
+  let!(:fake_proxy) { fake_ldap_sync_proxy(auth_hash.provider) }
+
+  before do
+    stub_ldap_config(
+        base: 'dc=example,dc=com',
+        active_directory: false
+    )
+  end
+
+  describe '#filter' do
+    it 'returns dns from an LDAP search' do
+      filter = '(ou=people)'
+
+      entry = ldap_group_entry(%w(john mary))
+      allow(fake_proxy.adapter).to receive(:ldap_search).and_return([entry])
+
+      expect(described_class.filter(fake_proxy, filter)).to eq('')
+    end
+  end
+end