chore: transform our tests from ESM to CJS to make them understandable by Jest

Previously, tests that use "import" were failing:
[ERROR]   ● Test suite failed to run
[ERROR]
[ERROR]     Jest encountered an unexpected token
[ERROR]
[ERROR]     This usually means that you are trying to import a file which Jest cannot parse, e.g. it's not plain JavaScript.
[ERROR]
[ERROR]     By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".
[ERROR]
[ERROR]     Here's what you can do:
[ERROR]      • If you are trying to use ECMAScript Modules, see https://jestjs.io/docs/en/ecmascript-modules for how to enable it.
[ERROR]      • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
[ERROR]      • If you need a custom transformation specify a "transform" option in your config.
[ERROR]      • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.
[ERROR]
[ERROR]     You'll find more details and examples of these config options in the docs:
[ERROR]     https://jestjs.io/docs/en/configuration.html
[ERROR]
[ERROR]     Details:
[ERROR]
[ERROR]     /Users/coder/git/mystamps/src/main/frontend/src/components/AddCommentForm.test.js:1
[ERROR]     ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){import { render, screen } from '@testing-library/react';
[ERROR]                                                                                                     ^
[ERROR]
[ERROR]     SyntaxError: Unexpected token {
[ERROR]
[ERROR]       at Runtime.createScriptFromCode (node_modules/jest-runtime/build/index.js:1350:14)

See also: https://github.com/ActuallyACat/jest-esm-transformer

Part of #1484

Author: php-coder

src/main/frontend/jest.config.js

@@ -2,6 +2,11 @@
 module.exports = {
 	// https://jestjs.io/docs/en/configuration#watchman-boolean
 	"watchman": false,
+	// required to use ECMAScript Modules (ESM)
+	// @todo #1484 Find a better way to use ESM with Jest and replace jest-esm-transformer
+	"transform": {
+		"\\.js$": "jest-esm-transformer"
+	},
 	// See: https://github.com/facebook/jest/issues/5064#issuecomment-401451361
 	// @todo #1484 Remove usage of jest-standard-reporter once facebook/jest#5064 is resolved
 	"reporters": [ "jest-standard-reporter" ]

src/main/frontend/package-lock.json

@@ -37,6 +37,7 @@
     "babel-jest": "~26.6.3",
     "babel-loader": "~8.1.0",
     "jest": "~26.6.3",
+    "jest-esm-transformer": "~1.0.0",
     "jest-standard-reporter": "~2.0.0",
     "react": "~16.8.6",
     "react-dom": "~16.8.6",