Add test for LDAP::Adapter#dn_matches_filter?

spec/lib/gitlab/ldap/ldap_adapter_spec.rb

+require 'spec_helper'
+
+describe Gitlab::LDAP::Adapter do
+  let(:adapter) { Gitlab::LDAP::Adapter.new }
+
+  describe :dn_matches_filter? do
+    let(:ldap) { double(:ldap) }
+    subject { adapter.dn_matches_filter?(:dn, :filter) }
+    before { adapter.stub(ldap: ldap) }
+
+    context "when the search is successful" do
+      context "and the result is non-empty" do
+        before { ldap.stub(search: [:foo]) }
+
+        it { should be_true }
+      end
+
+      context "and the result is empty" do
+        before { ldap.stub(search: []) }
+
+        it { should be_false }
+      end
+    end
+
+    context "when the search encounters an error" do
+      before { ldap.stub(search: nil) }
+
+      it { should be_false }
+    end
+  end
+end