Remove calling convention on FunctionRunner test

For this test to run on Windows, the explicit calling convention
must be removed so that the parser is free to use the host's
calling convention.

cranelift/filetests/src/function_runner.rs

@@ -99,7 +99,9 @@ mod test {
     #[test]
     fn nop() {
         let code = String::from(
-            "function %test() -> b8 system_v {
+            "
+            test run
+            function %test() -> b8 {
             ebb0:
                 nop
                 v1 = bconst.b8 true