[SPARK-31820][SQL][TESTS] Fix flaky JavaBeanDeserializationSuite

### What changes were proposed in this pull request?
Modified formatting of expected timestamp strings in the test `JavaBeanDeserializationSuite`.`testSpark22000` to correctly format timestamps with **zero** seconds fraction. Current implementation outputs `.0` but must be empty string. From SPARK-31820 failure:
- should be `2020-05-25 12:39:17`
- but incorrect expected string is `2020-05-25 12:39:17.0`

### Why are the changes needed?
To make `JavaBeanDeserializationSuite` stable, and avoid test failures like #28630 (comment)

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
I changed https://github.com/apache/spark/blob/7dff3b125de23a4d6ce834217ee08973b259414c/sql/core/src/test/java/test/org/apache/spark/sql/JavaBeanDeserializationSuite.java#L207 to
```java
new java.sql.Timestamp((System.currentTimeMillis() / 1000) * 1000),
```
to force zero seconds fraction.

Closes #28639 from MaxGekk/fix-JavaBeanDeserializationSuite.

Authored-by: Max Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/test/java/test/org/apache/spark/sql/JavaBeanDeserializationSuite.java

@@ -18,6 +18,8 @@
 package test.org.apache.spark.sql;
 
 import java.io.Serializable;
+import java.sql.Timestamp;
+import java.text.SimpleDateFormat;
 import java.time.Instant;
 import java.time.LocalDate;
 import java.util.*;
@@ -210,6 +212,17 @@ private static Row createRecordSpark22000Row(Long index) {
     return new GenericRow(values);
   }
 
+  private static String timestampToString(Timestamp ts) {
+    String timestampString = String.valueOf(ts);
+    String formatted = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(ts);
+
+    if (timestampString.length() > 19 && !timestampString.substring(19).equals(".0")) {
+      return formatted + timestampString.substring(19);
+    } else {
+      return formatted;
+    }
+  }
+
   private static RecordSpark22000 createRecordSpark22000(Row recordRow) {
     RecordSpark22000 record = new RecordSpark22000();
     record.setShortField(String.valueOf(recordRow.getShort(0)));
@@ -219,7 +232,7 @@ private static RecordSpark22000 createRecordSpark22000(Row recordRow) {
     record.setDoubleField(String.valueOf(recordRow.getDouble(4)));
     record.setStringField(recordRow.getString(5));
     record.setBooleanField(String.valueOf(recordRow.getBoolean(6)));
-    record.setTimestampField(String.valueOf(recordRow.getTimestamp(7)));
+    record.setTimestampField(timestampToString(recordRow.getTimestamp(7)));
     // This would figure out that null value will not become "null".
     record.setNullIntField(null);
     return record;