Convert test bundle to be an ES module

This should make it easier to speed up test runs in future by
pre-building npm dependencies into a separate bundle which the main test
bundle can import from.

rollup-tests.config.js

@@ -11,8 +11,7 @@ export default {
   input: 'build/scripts/test-inputs.js', // Input file generated by gulp task
   output: {
     file: 'build/scripts/tests.bundle.js',
-    format: 'iife',
-    name: 'testsBundle', // This just exists to suppress a build warning.
+    format: 'es',
     sourcemap: true,
   },
   treeshake: false,

src/karma.config.js

@@ -55,7 +55,7 @@ module.exports = function (config) {
       { pattern: './annotator/test/empty.html', watched: false },
 
       // Test bundles.
-      '../build/scripts/tests.bundle.js',
+      { pattern: '../build/scripts/tests.bundle.js', type: 'module' },
 
       // Sourcemaps for test bundles.
       { pattern: '../build/scripts/*.js.map', included: false },