Adds more multithreaded access testing to StructTest.

[tgregg]

Description of changes:

Adds multithreaded access testing to the relevant hand-written tests in StructTest and the generated test testRandomChanges().

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[popematt]

Will the guards in the ConcurrentHashMap and the synchronized list have any effects on the execution of the CompletableFutures?

If so, you might want to consider something like this, and have task i store its results directly into results[i].

Map<String, IonValue>[] results = new HashMap<String, IonValue>[numberOfTasks];

[tgregg]

Revision 2:

• Address Matt's comment by removing synchronization of results collection.
• Added a direct test for a scenario that happened infrequently in testRandomChanges. Also increased the number of iterations of testMultipleRandomChanges until this scenario was produced consistently. This scenario revealed an incorrect assertion in revision 1, which relied on IonStruct.get to return the same value for a given name in all circumstances, even when there were multiple mappings for that name. However, the documentation for IonStruct.get says "If the field name appears more than once, one of the fields will be selected arbitrarily". Therefore, I needed to add the assertStructContainsMapping function to allow for any value for the requested name to be considered acceptable.