Refactor mergeTestDescriptions() test utility to migrate to ESM (#7035

)

`mergeTestDescriptions()` is used by deprecated rules.
So I've created this commit from the `v16` branch, instead of the `main` branch.

This change enables the `esModuleInterop` flag to prevent the following TypeScript error:

```
lib/testUtils/mergeTestDescriptions.mjs:1:8 - error TS1259: Module '"/Users/masafumi.koba/git/stylelint/stylelint/node_modules/deepmerge/index"' can only be default-imported using the 'esModuleInterop' flag

1 import merge from 'deepmerge';
         ~~~~~

  node_modules/deepmerge/index.d.ts:20:1
    20 export = deepmerge;
       ~~~~~~~~~~~~~~~~~~~
    This module is declared with 'export =', and can only be used with a default import when using the 'esModuleInterop' flag.
```

lib/rules/at-rule-empty-line-before/__tests__/index.mjs

@@ -1,6 +1,6 @@
 import { stripIndent } from 'common-tags';
 
-import mergeTestDescriptions from '../../../testUtils/mergeTestDescriptions.js';
+import mergeTestDescriptions from '../../../testUtils/mergeTestDescriptions.mjs';
 import rule from '../index.js';
 const { messages, ruleName } = rule;
 

lib/rules/color-hex-length/__tests__/index.mjs

@@ -1,4 +1,4 @@
-import mergeTestDescriptions from '../../../testUtils/mergeTestDescriptions.js';
+import mergeTestDescriptions from '../../../testUtils/mergeTestDescriptions.mjs';
 import naiveCssInJs from '../../../__tests__/fixtures/postcss-naive-css-in-js.js';
 
 import rule from '../index.js';

lib/rules/comment-empty-line-before/__tests__/index.mjs

@@ -1,4 +1,4 @@
-import mergeTestDescriptions from '../../../testUtils/mergeTestDescriptions.js';
+import mergeTestDescriptions from '../../../testUtils/mergeTestDescriptions.mjs';
 import rule from '../index.js';
 const { messages, ruleName } = rule;
 

lib/testUtils/__tests__/mergeTestDescriptions.test.js → lib/testUtils/__tests__/mergeTestDescriptions.test.mjs

@@ -1,6 +1,4 @@
-'use strict';
-
-const mergeTestDescriptions = require('../mergeTestDescriptions');
+import mergeTestDescriptions from '../mergeTestDescriptions.mjs';
 
 describe('mergeTestDescriptions', () => {
 	it('merges empty objects', () => {

lib/testUtils/mergeTestDescriptions.mjs

@@ -0,0 +1,9 @@
+import merge from 'deepmerge';
+
+/**
+ * @param {...object} args
+ * @returns {object}
+ */
+export default function mergeTestDescriptions(...args) {
+	return merge.all(args);
+}