[vim9script] add test_override('defcompile', 1) option

[errael]

I'm doing some refactoring on a vim9script project that uses several files. I'm thinking of adding :defcompile to all the files temporarily to maximize checking.

While looking at import autoload I came across test_override('autoload', 1), which is good for testing script level references to possibly changed names/functions. But normal testing leaves some :def and :class unchecked.

If it's simple to do, with a defcompile option, the following

test_override('autoload', 1)
test_override('defcompile', 1)

would do a lot of checking, even for functions/methods that are only used in specialized circumstances, like error handling. This provides a manual way to get part of #13156.