test: fix nits in test-fs-mkdir-rmdir.js

* Add common.mustCall().
* Add check for error existence, not only for error details.
* Use test(), not match() in a boolean context.
* Properly reverse meanings of assert messages.

PR-URL: #13680
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-fs-mkdir-rmdir.js

@@ -24,14 +24,15 @@ fs.rmdirSync(d);
 assert(!common.fileExists(d));
 
 // Similarly test the Async version
-fs.mkdir(d, 0o666, function(err) {
+fs.mkdir(d, 0o666, common.mustCall(function(err) {
   assert.ifError(err);
 
-  fs.mkdir(d, 0o666, function(err) {
-    assert.ok(err.message.match(/^EEXIST/), 'got EEXIST message');
-    assert.strictEqual(err.code, 'EEXIST', 'got EEXIST code');
-    assert.strictEqual(err.path, d, 'got proper path for EEXIST');
+  fs.mkdir(d, 0o666, common.mustCall(function(err) {
+    assert.ok(err, 'got no error');
+    assert.ok(/^EEXIST/.test(err.message), 'got no EEXIST message');
+    assert.strictEqual(err.code, 'EEXIST', 'got no EEXIST code');
+    assert.strictEqual(err.path, d, 'got no proper path for EEXIST');
 
     fs.rmdir(d, assert.ifError);
-  });
-});
+  }));
+}));