test-macros: Fix default return type (#9469)

This commit fixes the handling of default return types in
`wasmtime-test-macros`. Prior to this commit, functions returning `()`
inserted a `()` after the return output, which resulted in a syntax
error.

This commit fixes this issue by quoting empty tokens when the return
type is the default one.

Author: saulecabrera

crates/test-macros/src/lib.rs

@@ -218,7 +218,7 @@ fn expand(test_config: &TestConfig, func: Fn) -> Result<TokenStream> {
         };
         let func_name = &func.sig.ident;
         let ret = match &func.sig.output {
-            ReturnType::Default => quote! { () },
+            ReturnType::Default => quote! {},
             ReturnType::Type(_, ty) => quote! { -> #ty },
         };
         let test_name = Ident::new(