Add new needs-backends tests annotations

Author: GuillaumeGomez

src/tests/directives.md

@@ -206,6 +206,7 @@ settings:
   `proc-macro` crate type.
 - `needs-target-std` — ignores if target platform does not have std support.
 - `ignore-backends` — ignores the listed backends, separated by whitespace characters.
+- `needs-backends` — only runs the test if current codegen backend is listed.
 
 The following directives will check LLVM support: