Add test for accessor docs of override properties (#4539)

[vmishenev]

Adds a regression test for #4539.

open class Parent {
    /** parent property docs */
    open var v = 0
}
class Child : Parent() {
    /** child property docs */
    override var v = 0
}

The generated getter/setter DFunctions of an override property must inherit the documentation from the override, not from the overridden parent property. The DProperty itself is already correct — only the accessors regressed (between 2.3.0-dev-425 and 2.3.0-dev-426).

This PR contains the test only.

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