Document and generalize DISABLE_DEMO as RUN_XFAIL.

I want to use this for a few other longstanding known failures.

This patch:

- Changes the variable names from disable demo to run xfail, for the
  sake of being general and avoiding double negatives everywhere.
- Changes the way the check is made to be consistent with SHOW_BROWSER
- Documents the variable in tests/README.md

tests/README.md

@@ -52,6 +52,27 @@ windows when debugging. You can do this by setting `SHOW_BROWSER=true`:
 
     SHOW_BROWSER=true make test
 
+## Dealing with tests which are expected to fail
+
+Some tests are known to fail, either always or intermittently. Obviously
+we should fix these, but so that the full test suite can remain useful
+in the interim, we disable these tests by default; if you want to run
+them you can set `RUN_XFAIL=true`:
+
+    RUN_XFAIL=true make test
+
+When writing tests, this variable is exposed as `run_xfail` in
+`tests/util.js`; you can disable a test by simply wrapping it in an
+if statement:
+
+```js
+if (utils.run_xfail) {
+  module.exports["Test something broken"] = function (browser) {
+    // ...
+  };
+}
+```
+
 ## How to dump the DOM for debugging
 
 Stick this in the test:

tests/run-local.sh

@@ -163,7 +163,7 @@ echo
 
 set +e
 
-export DISABLE_DEMO=true
+export RUN_XFAIL="${RUN_XFAIL:-false}"
 "$NPM" test
 
 cleanExit $?

tests/tests/basic.js

@@ -18,7 +18,7 @@
 
 var utils = require('../utils'),
     short_wait = utils.short_wait,
-    disable_demo = utils.disable_demo;
+    run_xfail = utils.run_xfail;
 
 module.exports = {
   "Test title" : function (browser) {
@@ -36,7 +36,7 @@ module.exports = {
       .end();
   },
 };
-if (!disable_demo) {
+if (run_xfail) {
   module.exports["Test demo login command"] = function (browser) {
     browser
       .loginDemo()

tests/utils.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var disable_demo = !!process.env.DISABLE_DEMO;
+var run_xfail = process.env.RUN_XFAIL !== "false";
 
 var wrapLoginDemo = function(test) {
   return function (browser) {
@@ -22,13 +22,13 @@ module.exports = {
   very_long_wait: 180000,
   default_width: 1280,
   default_height: 1024,
-  disable_demo: disable_demo,
+  run_xfail: run_xfail,
   testAllLogins: function (tests) {
     var newTests = {};
 
     var count = 0;
     var name, test;
-    if (!disable_demo) {
+    if (run_xfail) {
       for(name in tests) {
         test = tests[name];
         if (count === 0) {