RAD-972 Add docs for testing the Prebid.js library

See also: prebid/Prebid.js#421

Original version of these docs was written by @matthewlane, I just did
some edits/cleanup.  Thanks Matt!

_config.yml

@@ -80,6 +80,9 @@ nav_sections:
   - top_nav: dev_docs
     code: adaptors
     name: ADAPTORS
+  - top_nav: dev_docs
+    code: hacking
+    name: HACKING ON PREBID.JS
 
   - top_nav: overview
     code: intro

dev-docs/testing-prebid.md

@@ -0,0 +1,106 @@
+---
+layout: page
+title: Testing Prebid.js
+description: How to write tests for the Prebid.js library
+pid: 199
+
+top_nav_section: dev_docs
+nav_section: hacking
+hide: false
+---
+
+# Testing  Prebid.js
+{: .no_toc}
+
+Starting on 21 June 2016, all pull requests to the Prebid.js library will need to include tests with greater than 80% code coverage for any changed/added code before they can be merged into master.
+
+For more information, see Prebid.js issue [#421](https://github.com/prebid/Prebid.js/issues/421).
+
+This page describes how to test code in the Prebid.js repository to help prepare your pull request.
+
+* TOC
+{:toc}
+
+## Writing tests
+
+When you are adding code to Prebid.js, or modifying code that isn't covered by an existing test, test the code according to these guidelines:
+
+- If the module you are working on is already partially tested by a file within the `test` directory, add tests to that file
+- If the module does not have any tests, create a new test file
+- Group tests in a `describe` block
+- Test individual units of code within an `it` block
+- Within an `it` block, it may be helpful to use the "Arrange-Act-Assert" pattern
+  - _Arrange_: set up necessary preconditions and inputs
+    - e.g., creating objects, spies, etc.
+  - _Act_: call or act on the unit under test
+    - e.g., call the function you are testing with the parameters you set up
+  - _Assert_: check that the expected results have occurred
+    - e.g., use Chai assertions to check that the expected output is equal to the actual output
+- Test the public interface, not the internal implementation
+- If using global `pbjs` data structures in your test, take care to not completely overwrite them with your own data as that may affect other tests relying on those structures, e.g.:
+    - **OK**: `pbjs._bidsRequested.push(bidderRequestObject);`
+    - **NOT OK**: `pbjs._bidsRequested = [bidderRequestObject];`
+
+- When writing tests you may use ES2015 syntax if desired
+
+## Running tests
+
+After checking out the Prebid.js repository and installing dev dependencies with `npm install`, use the following commands to run tests as you are working on code:
+
+- `gulp test` will run the test suite once (`npm test` is aliased to call `gulp test`)
+- `gulp serve` will run tests once and stay open, re-running tests whenever a file in the `src` or `test` directory is modified
+
+## Checking results and code coverage
+
+Check the test results using these guidelines:
+
+- Look at the total number of tests run, passed, and failed (shown below the rainbow Nyan Cat in the shell window).
+- If all tests are passing, great.
+- Otherwise look for errors printed in the console for a description of the failing test.
+- You may need to iterate on your code or tests until all tests are passing.
+- Make sure existing tests still pass.
+- There is a table below the testing report that shows code coverage percentage, for each file under the `src` directory.
+- Each time you run tests, a code coverage report is generated in `build/coverage/lcov/lcov-report/index.html`.
+- This is a static HTML page that you can load in your browser.
+- On that page, navigate to the file you are testing to see which lines are being tested.
+- Red indicates that a line isn't covered by a test.
+- Gray indicates a line that doesn't need coverage, such as a comment or blank line.
+- Green indicates a line that is covered by tests.
+- The code you have added or modified must have greater than 80% coverage to be accepted.
+
+## Examples
+
+Prebid.js already has lots of tests. Read them to see how Prebid.js is tested, and for inspiration:
+
+- Look in `test/spec` and its subdirectories
+- Tests for bidder adaptors are located in `test/spec/adapters`
+
+A test module might have the following general structure:
+
+{% highlight js %}
+
+// Import or require modules necessary for the test, e.g.:
+import { assert } from 'chai';  // may prefer 'expect' in place of 'assert'
+import { adapter } from 'src/adapters/<adapter>';
+
+describe('<Adapter>', () => {
+
+  it('<description of unit or feature being tested>', () => {
+    // Arrange - set up preconditions and inputs
+    // Act - call or act on the code under test
+    // Assert - use chai to check that expected results have occurred
+  });
+
+  // Add other `describe` or `it` blocks as necessary
+
+});
+
+{% endhighlight %}
+
+## Resources
+
+The Prebid.js testing stack contains some of the following tools. It may be helpful to consult their documentation during the testing process.
+
+- [Mocha - test framework](http://mochajs.org/)
+- [Chai - BDD/TDD assertion library](http://chaijs.com/)
+- [Sinon - spy, stub, and mock library](http://sinonjs.org/)