cus-test fails from completion-category-overrides eglot adds

[Mattias Engdegård]

The test-custom-opts test currently fails because eglot adds elements to 
`completion-category-overrides` that doesn't match its declared type.

While eglot also adds to `completion-styles-alist`, from which the type of 
`completion-category-overrides` is derived, this happens too late; the 
defcustom type is already set.

Most likely that type needs to be made dynamic (late-bound) in some way. A 
validation function would be one way, but it doesn't help with interactive 
customisation, does it? (I'm no defcustom expert and it shows.)