These asserts aren't necessary since common.mustCall() already checks that the functions are called exactly the specified number of times (with 1 being the default).

Instead of using a fixed port as-is like this, consider using a custom http.Agent that returns an error in createConnection(). For an example of this, see this commit. This way we don't end up having a potentially flaky test in the future.

@mscdex I use not exists host instead.

assert is unused now.

These count variables and console.log()s are no longer needed either IMHO.

I don't think you have to include these first 21 lines in new files.

const req = http.get({ host }); should fit comfortably on one line.

You should probably verify that the error received here is the same one that is thrown. If you declare the error outside of the req.on('error', ...) block, but still throw it from in there, then you can just use assert.strictEqual() in here.