co19 tests check that allowed use of `void` is an error

[eernstg]

With #30176, the treatment of void in Dart programs is updated such that several usages that used to be compile-time errors are now allowed. In particular, void can be an actual type argument and it can be the type annotation of a variable/parameter and the return type of a getter.

This calls for changes to the following co19 tests:

• co19/Language/Types/Type_Void/syntax_t01: No longer an error.
• co19/Language/Types/Type_Void/syntax_t02: No longer an error.
• co19/Language/Types/Type_Void/syntax_t04: No longer an error.
• co19/Language/Types/Type_Void/syntax_t09: The declaration void v is no longer an error, so the documentation should be updated, but print(v) is a compile-time error.

[eernstg]

Closing this issue, it has been re-submitted as dart-lang/co19#120 in the co19 repository.