fix(types): forbid override modifier for functions without super-function

[i582]

Issue

Closes #1022.

Checklist

• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

[anton-trunov]

After fixing some test file names, let's also add tests with inheriting a defined function through a sequence of traits and then overriding it, something along these lines:

trait T1 { fun foo() { } }

trait T2 with T1 { }

trait T3 with T2 { }

contract Test with T3 { override fun foo() { } }

[anton-trunov]

Let's also add some diamond-like inheritance tests (if not added already)

[i582]

Let's also add some diamond-like inheritance tests (if not added already)

added