fibble encounters an error when using mocha v11 and node v20

[olsonpm]

Here's a reproduction

e.g.

$ git clone git@github.com:olsonpm/repro.git
$ cd repro
$ git checkout fibble-mocha11
$ npm ci
$ npx mocha test.js

  my module
    1) "before each" hook for "contains the stubbed stuff"

  0 passing (5ms)
  1 failing

  1) my module
       "before each" hook for "contains the stubbed stuff":
     TypeError [ERR_UNSUPPORTED_RESOLVE_REQUEST]: Failed to resolve module specifier "./mod.js" from "__PARENT_URL_PLACEHOLDER__": Invalid relative URL or base scheme is not hierarchical.

I found this when trying to debug a chain of other things - so I don't have a clue what the cause could be.