Fix test_subgroups - test as core feature. (#682)

* Fix test_subgroups - test as core feature.

* Fix kernels pragma in case of OpenCL 2.1 core subgroups

* Change global variable names to gVariable convention

* Allow subgroups tests execute 2 paths

 - as core feature
- as extension feature

* Fix code formatting.

* Set correct OpenCL version

* Fix code format

* test_subgroups review fixes:

* do not use global variables
* make IFP as separate tests
* use stringstream data type
* use special class to load function pointer for specific API

* Remove not not used variable

* Test subgroups - use common enums

* Test subgroups - set TEST_SKIPPED_ITSELF

test_conformance/subgroups/CMakeLists.txt

@@ -6,6 +6,7 @@ set(${MODULE_NAME}_SOURCES
     test_queries.cpp
     test_workitem.cpp
     test_workgroup.cpp
+    test_ifp.cpp
 )
 
 include(../CMakeCommon.txt)