test: ignore coverage to test nextTickFn

src/util/nextTick.ts

@@ -1,10 +1,13 @@
 type NextTickFn = (callback: () => void) => void;
+
+/* c8 ignore start */
 const nextTickFn =
   typeof setImmediate !== 'undefined'
     ? (setImmediate as NextTickFn)
     : typeof requestAnimationFrame !== 'undefined'
     ? (requestAnimationFrame as NextTickFn)
     : (setTimeout as NextTickFn);
+/* c8 ignore stop */
 
 export async function nextTickAsync() {
   return new Promise<void>((resolve) => nextTickFn(resolve));