compiletest: Allow only-unix lines in ui test headers

This is a valid variant, which can be confirmed by adding an invalid
variant such as `only-foobar`. In that case there will be an error later
in the parsing.

src/tools/compiletest/src/header.rs

@@ -810,6 +810,7 @@ const DIAGNOSTICS_DIRECTIVE_NAMES: &[&str] = &[
     "only-sparc",
     "only-sparc64",
     "only-thumb",
+    "only-unix",
     "only-wasm32",
     "only-wasm32-bare",
     "only-windows",