AbstractQueryTestCase is instanciated without mapping type one third
of the time. Since most of the tests are disabled when there is no type
this commit modifies the probability to use rarely() instead of a
fixed probability.

This commit changes the randomization to always create an index with a type.
It also adds a way to create a query shard context that maps to an index with
no type registered in order to explicitely test cases where there is no type.