Restructure the current MTR subquery tests, into a set of general subquery tests
that can be run with different settings of optimizer_switch and 
optimizer_join_cache_level.

Main goals:
 - Be able to more systematically test subquery execution with different settings
   of optimizer_switch.
 - Be able to incrementally add optimizer_switch sub-variables to a tree
   without having to operate with different result files in different trees.
   (This will make it easier to incrementally add original 6.0 features now
    residining in mysql-next-mr-opt-backporting to mysql-next-mr)
 - Be able have different defaults for optimizer_switch settings in different 
   trees without having to operate with different result files. 

Moved existing test files into include directory to be included from different
tests with different settings of optimizer_swicth.  Added tests for whether
switches and cache levels that tests depend upon are supported.  If not, tests
are skipped.

Added result ordering to some queries to avoid result differences when switches
are turned off.