Update mongodb-memory-server createBucket regression tests

Expect BucketAlreadyExists on concurrent metastore races while keeping
the pre-existing collection regression when createCollection is
idempotent.

Issue: ARSN-613

Author: benzekrimaha

tests/unit/storage/metadata/mongoclient/MongoClientInterface.spec.js

@@ -644,7 +644,7 @@ describe('MongoClientInterface, tests', () => {
     });
 
     it('should create a bucket whose backing collection already exists', done => {
-        const bucketName = 'test-bucket-collection-exists';
+        const bucketName = `test-bucket-collection-exists-${Date.now()}`;
         async.waterfall(
             [
                 next => {
@@ -666,13 +666,22 @@ describe('MongoClientInterface, tests', () => {
         );
     });
 
-    it('should succeed on concurrent createBucket calls for the same bucket', done => {
+    it('should return BucketAlreadyExists on concurrent createBucket calls', done => {
         const bucketName = 'test-bucket-concurrent-create';
+        const results = [];
         async.times(
             5,
-            (n, next) => createBucket(client, bucketName, false, next),
+            (n, next) =>
+                createBucket(client, bucketName, false, err => {
+                    results.push(err);
+                    next();
+                }),
             err => {
                 assert.ifError(err);
+                const successes = results.filter(e => !e).length;
+                const alreadyExists = results.filter(e => e?.is?.BucketAlreadyExists).length;
+                assert.strictEqual(successes, 1);
+                assert.strictEqual(alreadyExists, 4);
                 client.deleteBucket(bucketName, logger, done);
             },
         );