fmt: add function with variadic arg to test file

vlib/v/fmt/tests/simple_expected.vv

@@ -182,3 +182,7 @@ fn fn_with_match_expr() {
 		else {}
 	}
 }
+
+fn fn_variadic(arg int, args ...string) {
+	println('Do nothing')
+}

vlib/v/fmt/tests/simple_input.vv

@@ -182,3 +182,7 @@ fn fn_with_match_expr() {
 		else {}
 	}
 }
+
+fn fn_variadic(arg int, args... string) {
+	println('Do nothing')
+}