Make sure to actually use yarn to install the main-packages, otherwise the packages.json#resolutions property will not be used and @babel/helpers would not get overruled

Overruled @babel/helpers to fix how initializers play with decorated properties. Thus circumventing the imperformant and lengthy code being generated by babel in the non-legacy option.

packages/babel-helpers/test/helpers/define-helper.js

@@ -0,0 +1,26 @@
+import path from "path";
+import template from "@babel/template";
+import helpers from "../../lib/helpers";
+
+function getHelperId(dir, name) {
+  const testName = path.basename(dir);
+  return `_$_${testName}_${name}`;
+}
+
+export default function defineHelper(
+  dir: string,
+  name: string,
+  code: string,
+): string {
+  const id = getHelperId(dir, name);
+  if (id in helpers) {
+    throw new Error(`The ${id} helper is already defined.`);
+  }
+  Object.defineProperty(helpers, id, {
+    value: {
+      minVersion: "7.0.0-beta.0",
+      ast: template.program(code),
+    },
+  });
+  return id;
+}