Change how parallel mode is tested

packages/hardhat-core/test/builtin-tasks/test.ts

@@ -38,7 +38,7 @@ describe("test task", function () {
     describe("with the default config", function () {
       useEnvironment();
 
-      it("should fail in serial mode because the HRE is shared between test files", async function () {
+      it("should fail in serial mode", async function () {
         await this.env.run("test", {
           noCompile: true,
         });
@@ -47,7 +47,7 @@ describe("test task", function () {
         (process as any).exitCode = undefined;
       });
 
-      it("should pass in parallel mode because the HRE is not shared between test files", async function () {
+      it("should pass in parallel mode", async function () {
         await this.env.run("test", {
           noCompile: true,
           parallel: true,

packages/hardhat-core/test/fixture-projects/test-task/parallel-tests/test/check-parallel.js

@@ -0,0 +1,7 @@
+const assert = require("assert");
+
+describe("parallel tests", function () {
+  it("pass if run in parallel mode", async function () {
+    assert(process.env.MOCHA_WORKER_ID !== undefined);
+  });
+});