Fix: using custom interface in parallel mode

[juergba]

Description

see discussion in #4659, thanks to @love-expressen for his analysis.

When using the --ui option together with --parallel the ui option is not passed to the workers. This causes errors when running test files that depend on the UI being set up in the worker executing the test.

The root cause of the issue is that mocha refers to external UI:s in the options object as function references and not the UI identifier string. When the options object is being serialized all functions are (thankfully) ignored. This causes the mocha instance in the workers to default to the bdd UI.

Description of the Change

Mocha's native interfaces are cached in Mocha.interfaces as function references where each key corresponds to an interface name. The interface name can easily be serialized and passed to the parallel workers.
Now we use an identical approach for third party interfaces and add them to Mocha.interfaces.

Applicable issues

closes #4659