Use `verification-helper:` comments instead of `.test.cpp` extensions to distinguish verification files

[kmyk]

Description / 説明

The current implementation uses file extensions like .test.cpp to distinguish whether the files are library files or verification files.

We want to use the special comment verification-helper: URL instead to mark the file as a verification file.

Motivation / 動機

The approach with comments allows users to use arbitrary file names. This is important for some languages like Java.

Other notes / その他

• 拡張子に意味を持たせちゃったのは、実装初期に私がした判断ミスのひとつだと認識している
• 実装はやるだけだけど、互換性は壊しちゃだめだし変更もきちんと周知しないとだめなので丁寧さが要求される
• Will you try to create a pull request?
  • no