test: mark TestHelper as causing side-effects

It adds chai matchers, so it is.

test/config/karma.unit.js

@@ -59,6 +59,10 @@ module.exports = function(karma) {
       mode: 'development',
       module: {
         rules: [
+          {
+            test: require.resolve('../TestHelper.js'),
+            sideEffects: true
+          },
           {
             test: /\.css|\.bpmn$/,
             use: 'raw-loader'