Stabeilze SearchStatsTests

Query stats are only present (not 0) on nodes that hold a shard of the index.
Fix AbstractSimpleEngineTests versioning tests.

Version is now stored on a distinct field, that AbstractSimpleEngineTests
didn't correctly add before running tests. This generated a test failure
when the version needed to be loaded from the index.
Stabelize more failing tests.

- SimpleSortTests#testSortScript which was not using the mapping correctly
- SearchStatsTests#testSimpleStats which didn't clear the stats before
  running the test and a previous run could have added queries
Wfst suggester prototype