Merge pull request #2129 from reitermarkus/spec-doctor

Convert `brew doctor` test to spec.

Library/Homebrew/test/cmd/doctor_spec.rb

+describe "brew doctor", :integration_test do
+  specify "check_integration_test" do
+    expect { brew "doctor", "check_integration_test" }
+      .to output(/This is an integration test/).to_stderr
+  end
+end

Library/Homebrew/test/doctor_test.rb

-require "testing_env"
-
-class IntegrationCommandTestDoctor < IntegrationCommandTestCase
-  def test_doctor
-    assert_match "This is an integration test",
-                 cmd_fail("doctor", "check_integration_test")
-  end
-end