Scaladoc: add tests for `extension` signatures

[KacperFKorban]

Scaladoc should test signatures of extensions just like it's done for defs. This way we can prevent issues like #16647

The appropriate file for this is most likely:
https://github.com/lampepfl/dotty/blob/main/scaladoc/test/dotty/tools/scaladoc/signatures/SignatureTest.scala

[Dedelweiss]

Hello, I would like to work on this issue. From what I understand the SignatureTest.scala file, it is linked to the TranslatableSignaturesTestCases.scala file which allows to call different scala files such as extensionParams.scala which contains examples of extension methods to test. So, is it enough to add new examples in this file (or another one), or do we need to modify the SignatureTest.scala file?

[KacperFKorban]

@Dedelweiss I think that SignatureTest.scala will have to be modified to add checking for extension signatures. In TranslatableSignaturesTestCases.scala there already should exist more than enough tests for extension methods. The problem is that for those tests only the def part is tested to be present in generated scaladoc sites which allowed for bugs like #16647 to slip through. (where only the extension (...) part was wrongly generated)