Docs: Add adr about running QUnit tests in node

Change-Id: Ie887a88b81dcc120e5e9bf53c9c0bf5886259a2e

doc/adr/0007-prefer-running-qunit-tests-in-node-js.md

@@ -0,0 +1,47 @@
+# 7. Prefer running QUnit tests in node.js
+
+Date: 08/03/2017
+
+## Status
+
+Accepted
+
+## Context
+
+QUnit tests were set up to run on a browser or phantom.js on top of a running
+mediawiki server.
+
+We want to:
+* Speed up the test running in a development cycle
+  * Full page reload every time you change tests or source takes too long
+* Leverage common.js in our tests to load our sources and other test helpers
+  like stubs
+  * Remove global variables
+
+## Decision
+
+QUnit tests will be migrated from `tests/qunit/ext.popups/` to
+`tests/node-qunit/` and will be run by the script `npm run test:node` in a node
+environment with access to QUnit, a fake jsdom, and jQuery. Powered by the node
+package `mw-node-qunit`.
+
+Node tests will be run in CI too in the npm job that runs `npm test`.
+
+It is preferred to use the node-qunit test environment, as it is fast,
+leverages common.js modules, and isolated.
+
+When impossible because of extreme coupling with mediawiki.js, put the
+**integration** tests in `tests/qunit/`.
+
+## Consequences
+
+* README has instructions on how to run different types of tests
+* Node based QUnit tests...
+  * can be run independently of mediawiki
+  * are isolated and need to stub global variables used in sources
+  * are run in node.js 6+ which runs in CI on wikimedia infrastructure
+    * which means they leverage that platforms advantages (loading files with
+      common.js, using es6, using the node stdlib if necessary)
+* Most tests are migrated to `tests/node-qunit` and only the ones that strictly
+  need it remain in `tests/qunit` (`processLinks.test.js` for example, see
+  comment in file)