test, util: refactor isObject in test-util

refactor the test for isObject function including falsy values,
the use of strictEquals and the format actual, expected

PR-URL: #28878
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-util.js

@@ -73,7 +73,11 @@ assert.strictEqual(util.isError([]), false);
 assert.strictEqual(util.isError(Object.create(Error.prototype)), true);
 
 // isObject
-assert.ok(util.isObject({}) === true);
+assert.strictEqual(util.isObject({}), true);
+assert.strictEqual(util.isObject([]), true);
+assert.strictEqual(util.isObject(new Number(3)), true);
+assert.strictEqual(util.isObject(Number(4)), false);
+assert.strictEqual(util.isObject(1), false);
 
 // isPrimitive
 assert.strictEqual(util.isPrimitive({}), false);