feat(tests): Support an additionalScripts option

This is a list of scripts to load (in order) once the required modules
have been bootstrapped.

We do this using goog.addDependency to make the first script depend
on the required modules, then each subsequent script depend on the
previous one, and then finally goog.bootstrapping the last such script.

tests/bootstrap.js

@@ -80,6 +80,11 @@
       'php_compressed.js',
       'python_compressed.js',
     ],
+
+    // Additional scripts to be loaded after Blockly is loaded,
+    // whether Blockly is loaded from compressed or uncompressed.
+    additionalScripts: [
+    ],
   };
   if (typeof window.BLOCKLY_BOOTSTRAP_OPTIONS === 'object') {
     Object.assign(options, window.BLOCKLY_BOOTSTRAP_OPTIONS);
@@ -125,11 +130,21 @@
         '  window.BlocklyMsg = window.Blockly.Msg;\n' +
         '  delete window.Blockly;\n' +
         '</script>\n');
-    const allRequires = options.requires.map(quote).join();
+    const requiresString = options.requires.map(quote).join();
+    const scriptsString = options.additionalScripts.map(quote).join();
     document.write(
         '<script>\n' +
+        '  let requires = [' + requiresString + '];\n' +
+        '  let scripts = [' + scriptsString + '];\n' +
+        '  for (const script of scripts) {\n' +
+        '    const fakeModuleName = \n' +
+        '        "script." + script.replace(/[./]/g, "-");\n' +
+        '    goog.addDependency("../../" + script, [fakeModuleName], \n' +
+        '        requires, {"lang": "es6"});\n' +
+        '    requires = [fakeModuleName];\n' +
+        '  }\n' +
         '  window.BlocklyLoader = new Promise((resolve, reject) => {\n' +
-        '    goog.bootstrap([' + allRequires + '], resolve);\n' +
+        '    goog.bootstrap(requires, resolve);\n' +
         '  }).then(() => {\n' +
         '    // Copy Messages from temp Blockly.Msg object to the real one:\n' +
         '    Object.assign(goog.module.get(\'Blockly\').Msg,\n' +
@@ -141,7 +156,8 @@
   } else {
     // We need to load Blockly in compressed mode.  Load
     // blockly_compressed.js et al. using <script> tags.
-    const scripts = options.compressedScripts;
+    const scripts =
+        options.compressedScripts.concat(options.additionalScripts);
     for (let i = 0; i < scripts.length; i++) {
       document.write(
           '<script src="' + options.root + scripts[i] + '"></script>');