switch to esmock, re #221

[iambumblehead]

Fixes #221

I hope the changes here are OK and wanted to mention two things. One thing, some tests, expected to fail, were passing in my local environment. I found this this recent commit which indicated to me that the passing tests might be OK -- not sure. The tests in question are found in test/notifier.js.

The other thing is, esmock-imported modules are each using their own state, so tests could be updated in the future to run concurrently rather than sequentially (here).

[sindresorhus]

CI is failing

[iambumblehead]

Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'clear-module' imported from /home/runner/work/update-notifier/update-notifier/test/fs-error.js

I'll resolve it in a moment

[iambumblehead]

its ready

[sindresorhus]

Thanks :)