assert: ensure .rejects() disallows sync throws

This updates `assert.rejects()` to disallow any errors that are thrown
synchronously from the given function. Previously, throwing an error
would cause the same behavior as returning a rejected Promise.

Fixes: #19646

lib/assert.js

@@ -451,8 +451,11 @@ async function waitForActual(block) {
   if (typeof block !== 'function') {
     throw new ERR_INVALID_ARG_TYPE('block', 'Function', block);
   }
+
+  // Return a rejected promise if `block` throws synchronously.
+  const resultPromise = block();
   try {
-    await block();
+    await resultPromise;
   } catch (e) {
     return e;
   }

test/parallel/test-assert-async.js

@@ -13,7 +13,7 @@ common.crashOnUnhandledRejection();
 
 (async () => {
   await assert.rejects(
-    () => assert.fail(),
+    async () => assert.fail(),
     common.expectsError({
       code: 'ERR_ASSERTION',
       type: assert.AssertionError,
@@ -57,4 +57,17 @@ common.crashOnUnhandledRejection();
       }
     );
   }
+
+  {
+    const THROWN_ERROR = new Error();
+
+    await assert.rejects(() => {
+      throw THROWN_ERROR;
+    }).then(common.mustNotCall())
+      .catch(
+        common.mustCall((err) => {
+          assert.strictEqual(err, THROWN_ERROR);
+        })
+      );
+  }
 })().then(common.mustCall());