Use a serial merge scheduler to avoid race conditions when checking for hasPendingMerges().

Closes #12763

Author: jpountz

lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java

@@ -2372,7 +2372,10 @@ public void testCorruptFirstCommit() throws Exception {
 
   public void testHasUncommittedChanges() throws IOException {
     Directory dir = newDirectory();
-    IndexWriter writer = new IndexWriter(dir, newIndexWriterConfig(new MockAnalyzer(random())));
+    IndexWriter writer = new IndexWriter(dir,
+        newIndexWriterConfig(new MockAnalyzer(random()))
+        // Use a serial merge scheduler to avoid race conditions when checking for hasPendingMerges()
+        .setMergeScheduler(new SerialMergeScheduler()));
     assertTrue(
         writer.hasUncommittedChanges()); // this will be true because a commit will create an empty
     // index