[lake/paimon] Fix timestamp field assertion in test

- Use getTimestamp() instead of getLong() for timestamp system column
- Addresses review comment: 'should assert getTimestamp because this is a timestamp type'

This completes all review feedback from wuchong in PR apache#2189

Author: buvb

fluss-lake/fluss-lake-paimon/src/test/java/org/apache/fluss/lake/paimon/tiering/FlussRecordAsPaimonRowTest.java

@@ -635,7 +635,8 @@ void testPaimonSchemaWiderThanFlussRecord() {
         assertThat(flussRecordAsPaimonRow.isNullAt(1)).isTrue();
         assertThat(flussRecordAsPaimonRow.getInt(2)).isEqualTo(tableBucket);
         assertThat(flussRecordAsPaimonRow.getLong(3)).isEqualTo(logOffset);
-        assertThat(flussRecordAsPaimonRow.getLong(4)).isEqualTo(timeStamp);
+        assertThat(flussRecordAsPaimonRow.getTimestamp(4, 3))
+                .isEqualTo(Timestamp.fromEpochMillis(timeStamp));
     }
 
     @Test