Fix an issue and add relevant tests when describe and xdescribe fail …

…when not supplied with a callback (issue mochajs#1744).

test/integration/fixtures/suite/describe.callback.js

@@ -0,0 +1 @@
+describe('a suite without a callback');

test/integration/fixtures/suite/xdescribe.callback.js

@@ -0,0 +1 @@
+xdescribe('a pending suite without a callback');

test/integration/suite.js

@@ -0,0 +1,29 @@
+var assert = require('assert');
+var run    = require('./helpers').runMocha;
+var args   = [];
+
+describe('.describe()', function() {
+  this.timeout(1000);
+  it('should throw a helpful error message when a callback for describe is not supplied', function(done) {
+    run('suite/describe.callback.js', args, function(err, res) {
+      assert(!err);
+      pattern = new RegExp('TypeError: a callback is not supplied', 'g');
+      var result = res.output.match(pattern) || [];
+      assert.equal(result.length, 1);
+      done();
+    });
+  });
+});
+
+describe('.xdescribe()', function() {
+  this.timeout(1000);
+  it('should not throw an error when a callback for xdescribe is not supplied', function(done) {
+    run('suite/xdescribe.callback.js', args, function(err, res) {
+      assert(!err);
+      pattern = new RegExp("Error", 'g');
+      var result = res.output.match(pattern) || [];
+      assert.equal(result.length, 0);
+      done();
+    });
+  });
+});