Prepare existing JUnit tests for running with automatic index statistics generation

During prototyping of a mechanism to create and update index statistics automatically (see DERBY-4771) it turned out that some of the tests fail due to the presence of [improved] index statistics.
Where appropriate the tests should be adjusted to cope with this background activity, and where not appropriate it should be noted such that the feature, when introduced, can be disabled.

As part of this effort I will introduce a utility class for dealing with index statistics in tests. These will be used by some existing tests and also the new tests that will be written for the new feature.

At this point two tests requiring adjustments have been identified (OrderByAndSortAvoidance, UpdateStatisticsTest), and one is under investigation (AutoIncrementTest). More may surface as the work on the feature progress.