add has_offload/needs-offload to the test infra

Author: ZuseZ4

src/tests/directives.md

@@ -213,6 +213,7 @@ settings:
   that this directive can be overriden with the `--bypass-ignore-backends=[BACKEND]` command line
   flag. 
 - `needs-backends` — only runs the test if current codegen backend is listed.
+- `needs-offload` — ignores if our LLVM backend was not built with offload support.
 
 The following directives will check LLVM support: