Karma: fix warning by ignoring olm

If olm is not installed, the webpack build for the karma tests gives an ugly
error. None of the tests currently care if olm is installed or not, so fix this
for now by just ignoring the olm module.

karma.conf.js

@@ -131,6 +131,11 @@ module.exports = function (config) {
                     path.resolve('./test'),
                 ],
             },
+            plugins: [
+                // olm may not be installed, so avoid webpack warnings by
+                // ignoring it.
+                new webpack.IgnorePlugin(/^olm$/),
+            ],
             devtool: 'inline-source-map',
         },
     });