--require esm works as command-line arg, but not in mocha.opts

[alexbepple]

I believe this is a regression in Mocha 6 from Mocha 5. At least, I have not found any doc describing the change in behavior.

Here is a minimal repo demonstrating the difference in behavior.
https://github.com/alexbepple/possible-mocha-bug-with-require

I believe this to be a bug.

[juergba]

@alexbepple thank you for this issue. Yes, it's a bug, see #3975.
We have already fixed it in current master, but it's not published, yet.

BTW we recommend not to use mocha.opts any longer and to switch to our configuration files. mocha.opts is going to be deprecated in the next major release.