[bugfix] resolve 'ERR_REQUIRE_ESM' error occurring on 'npm run test'

[atla5]

Background

• this is a configuration problem that started occurring, to my knowledge, without any major code change on our end.
• we're using the open-wc testing configuration described in their documentation with a single-line change to include our bundle.css in the files

Description of the Bug

• running npm run test no longer runs the karma testing suite as it used to and prints an error related to ERR_REQUIRE_ESM

Output

$ npm run test
$ karma start karma.conf.js
07 05 2020 16:05:24.564:ERROR [config]: Error in config file!
  Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: /Users/aidans/projects/wc/karma.conf.js
require() of ES modules is not supported.
require() of /Users/aidans/projects/wc/karma.conf.js from /usr/local/lib/node_modules/karma/lib/config.js is an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which defines all .js files in that package scope as ES modules.
Instead rename karma.conf.js to end in .cjs, change the requiring code to use import(), or remove "type": "module" from /Users/aidans/projects/wc/package.json.

    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1172:13)
    at Module.load (internal/modules/cjs/loader.js:1000:32)
    at Function.Module._load (internal/modules/cjs/loader.js:899:14)
    at Module.require (internal/modules/cjs/loader.js:1042:19)
    at require (internal/modules/cjs/helpers.js:77:18)
    at Object.parseConfig (/usr/local/lib/node_modules/karma/lib/config.js:357:22)
    at new Server (/usr/local/lib/node_modules/karma/lib/server.js:67:24)
    at Object.exports.run (/usr/local/lib/node_modules/karma/lib/cli.js:295:7)
    at Object.<anonymous> (/usr/local/lib/node_modules/karma/bin/karma:3:23)
    at Module._compile (internal/modules/cjs/loader.js:1156:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1176:10)
    at Module.load (internal/modules/cjs/loader.js:1000:32)
    at Function.Module._load (internal/modules/cjs/loader.js:899:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:74:12)
    at internal/main/run_main_module.js:18:47

Attempted Fixes

1. remove "type": "module" from package.json -> no tests are run, they just hang.

$ karma start
Browserslist: caniuse-lite is outdated. Please run next command `npm update`

START:
07 05 2020 16:00:58.237:INFO [karma-server]: Karma v4.4.1 server started at http://0.0.0.0:9876/
07 05 2020 16:00:58.238:INFO [launcher]: Launching browsers ChromeHeadlessNoSandbox with concurrency unlimited
07 05 2020 16:00:58.243:INFO [launcher]: Starting browser ChromeHeadless
07 05 2020 16:00:58.606:INFO [HeadlessChrome 81.0.4044 (Mac OS X 10.15.4)]: Connected on socket l9XHWr6ckrpTRUrqAAAA with id 67019134
Resolved to invalid user agent: {"family":"Chrome","version":"81.0.4044"}. Using max compatibility mode. Try updating your dependencies, or file an issue with a reproduction.

07 05 2020 16:01:28.609:WARN [HeadlessChrome 81.0.4044 (Mac OS X 10.15.4)]: Disconnected (0 times), because no message in 30000 ms.
HeadlessChrome 81.0.4044 (Mac OS X 10.15.4) ERROR
  Disconnected, because no message in 30000 ms.
07 05 2020 16:01:28.612:WARN [framework.snapshot]: Snapshot data is unavailable

Finished in 30.002 secs / 0 secs @ 16:01:28 GMT-0400 (Eastern Daylight Time)

2. rename the karma.conf.js to karma.conf.cjs and update command to karma start karma.conf.cjs -> no tests are run, and they just hang. even after npm update

$ karma start karma.conf.cjs
Browserslist: caniuse-lite is outdated. Please run next command `npm update`

START:
07 05 2020 16:04:49.810:INFO [karma-server]: Karma v4.4.1 server started at http://0.0.0.0:9876/
07 05 2020 16:04:49.811:INFO [launcher]: Launching browsers ChromeHeadlessNoSandbox with concurrency unlimited
07 05 2020 16:04:49.816:INFO [launcher]: Starting browser ChromeHeadless
07 05 2020 16:04:50.267:INFO [HeadlessChrome 81.0.4044 (Mac OS X 10.15.4)]: Connected on socket -prvuOvGc-8ilzMLAAAA with id 98024656
Resolved to invalid user agent: {"family":"Chrome","version":"81.0.4044"}. Using max compatibility mode. Try updating your dependencies, or file an issue with a reproduction.

^C

3. try adding --experimental-modules flag for node -> no effect

Steps to Reproduce

$ npm i
$ npm run test

Screenshots