Add test/HOWTO.md (impress#696)

Note: I will now expect new PRs to add tests for the functionality they
add or change. (Including my own, mostly...)

Fixes impress#657

test/HOWTO.md

@@ -0,0 +1,67 @@
+Testing HowTo
+=============
+
+Install and run tests
+---------------------
+
+    npm install
+    npm run test
+    npm run lint
+
+QUnit is used for unit tests. Above npm command will run them under karma. You can also run the
+same tests as plain qunit in Firefox (but not Chrome, due to use of iframe):
+
+    firefox qunit_test_runner.html
+
+For linting both jshint and jscs are used. As is customary, they are configured via
+[.jshintrc](.jshintrc) and [.jscsrc](.jscsrc).
+
+
+Anatomy of a unit test
+----------------------
+
+Impress.js likes to use the entire browser window, sets classes on the body element, and so on. For
+this reason, we use an iframe to run each test. QUnit tests are supposed to run inside a
+`<div id="qunit-fixture">` element. So in our case, the contents of that div is an iframe.
+
+Each test consists of 2 files: A html file that contains a normal impress.js presentation, and
+a js file that contains your QUnit tests. For example, see
+[test/core_tests_presentation.html](test/core_tests_presentation.html) and
+[test/core_tests.js](test/core_tests.js). Note that the QUnit tests run in the parent window.
+
+[test/helpers.js](test/helpers.js) contains helper functions to create the iframe and load the
+html file that contains the impress.js presentation to be tested.
+
+An example test would therefore look like:
+
+    QUnit.test( "Example tests", function( assert ) {
+      loadIframe( "test/core_tests_presentation.html", assert, function() {
+        initPresentation( assert, function() {
+          var iframe = document.getElementById( "presentation-iframe" );
+          var iframeDoc = iframe.contentDocument;
+          var iframeWin = iframe.contentWindow;
+          var step1 = iframeDoc.querySelector( "div#step-1" );
+
+          assert.equal( step1.dataset.x, "0", "data-x attribute set to zero" );
+          assert.equal( step1.dataset.y, "0", "data-y attribute set to zero" );
+
+
+Where to save unit tests?
+-------------------------
+
+Tests related to a plugin are saved in the plugin folder. See for example
+[src/plugins/navigation/navigation_tests.js](src/plugins/navigation/navigation_tests.js).
+
+Tests for impress.js core, or other tests not related to one specific plugin, go under
+[test/](test/).
+
+Adding your js file to the right places
+---------------------------------------
+
+There are 3 files where you need to add your new test so that it gets run. (Yeah, see
+[#658](https://github.com/impress/impress.js/issues/658) for more on that topic...)
+
+1. qunit_test_runner.html
+2. karma.conf.js
+3. karma.conf-sauce.js
+

test/helpers.js

@@ -12,7 +12,7 @@ var loadIframe = function( src, assert, callback ) {
   console.log( 'Begin loadIframe' );
 
   // When running in Karma, the #qunit-fixture appears from somewhere and we can't set its
-  // contents in advance.
+  // contents in advance, so we set it now.
   var fix = document.getElementById( 'qunit-fixture' );
   fix.innerHTML = [
     '\n',