Use LinkedHashMap in TestStandardObjectInspectors.java

The test in org.apache.hadoop.hive.serde2.objectinspector.TestStandardObjectInspectors#testStandardUnionObjectInspector can fail due to a different iteration order of HashMap. The failure is presented as follows.

org.junit.ComparisonFailure:
expected:<{4:{6:"six",7:"seven",8:"eight"}}>
but was:<{4:{6:"six",8:"eight",7:"seven"}}>

The reason is that the assertion assertEquals("{4:{6:\"six\",7:\"seven\",8:\"eight\"}}", SerDeUtils.getJSONString(union, uoi1)); compares a hard-coded string against the string representation of a JSON object, which is implemented by a HashMap. To get the string, the HashMap is iterated here at serde/src/java/org/apache/hadoop/hive/serde2/SerDeUtils.java:343
for (Object entry : omap.entrySet())

The specification about HashMap says that "this class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time". The documentation is here for your reference: https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html

The fix is to use LinkedHashMap instead of HashMap. In this way, the non-deterministic behaviour is eliminated and the test will become more stable.