Remove demo/test CGI modules (hello, abend0c1, test, httptest)

[mgrossmann]

Remove the following demo/test CGI source files from the repository:

• src/hello.c
• src/abend0c1.c
• src/test.c
• src/httptest.c

Also remove any CGI references to these modules from samplib/httpprm0 if present.

Remove the corresponding [[link.module]] entries from project.toml (HELLO, ABEND0C1, HTTPTEST).

Verify with a clean build + link.