spec: preserve stderr from the browser, refs SD-2148

test plan:
1. see previous patchset build where Xvfb was intentionally killed
2. note the `ERROR:browser_main_loop.cc(272)] Gtk: cannot open display: :20.0`

Change-Id: Ia7bd7f82e85972c25fe7221cb2441473a3267936
Reviewed-on: https://gerrit.instructure.com/106517
Tested-by: Jenkins
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>
QA-Review: Landon Wilkins <lwilkins@instructure.com>

karma.debugging.js

@@ -0,0 +1,24 @@
+/* preserve stderr from chrome/etc., so we can know why it fails, e.g.
+ *
+ *   [7644:7644:0327/202136.818897:ERROR:browser_main_loop.cc(272)] Gtk: cannot open display:
+ *
+ * this is a bit hacky... karma DI plugin stuff is magical and crazy 'n
+ * all, but it doesn't let you override ProcessLauncher stuff (unless you
+ * also want to copypasta all of karma-chrome-launcher and more). so
+ * just override this one thing before karma proper loads up
+ */
+
+const ProcessLauncher = require('karma/lib/launchers/process')
+const tempDir = require('karma/lib/temp_dir')
+const { spawn } = require('child_process')
+
+ProcessLauncher.decoratorFactory = function (timer) {
+  return function (launcher, processKillTimeout) {
+    const spawnWithStderrPassthrough = function (command, args = [], options = {}) {
+      const newOptions = Object.assign({}, options)
+      newOptions.stdio = ['ignore', 'ignore', 'inherit']
+      return spawn.call(null, command, args, newOptions)
+    }
+    ProcessLauncher.call(launcher, spawnWithStderrPassthrough, tempDir, timer, processKillTimeout)
+  }
+}

package.json

@@ -112,7 +112,7 @@
   },
   "repository": "instructure/canvas-lms",
   "scripts": {
-    "test": "karma start --single-run",
+    "test": "node -r ./karma.debugging.js node_modules/.bin/karma start --single-run",
     "test-watch": "karma start",
     "preinstall": "script/gem_npm install",
     "preupdate": "script/gem_npm update",