Introduce External/OpenCL subsuite prototype

[kasaurov]

This is a prototype of OpenCL External sub suite to test OpenCL tests. Several AMDAPP SDK samples are added as examples.
The sub-suite uses folder hierarchy to combine tests into logical groups:

tests\
    AMDAPP_SDK\
        BufferBandwidth\
        HelloWorld/
        Template/
    TestSuite1\
        test_1\
        test_2\
    Test_AAA\
...        

The CMakeList.txt on sub-suite or test level allows to use specific ways to build/run tests if needed. This makes the top level CMakeList.txt simple and easy to read.

Build/Run targets support general and specific targets:

opencl-tests-simple
opencl-tests-simple-BufferBandwidth
check-opencl-simple
check-opencl-simple-BufferBandwidth

To build/run tests see External/OpenCL/README.

TODO List:

1. Update the individual test targets name to automatically include sub-suite[s] name: check-opencl-simple-BufferBandwidth -> check-opencl-simple-AMDAPP_SDK-BufferBandwidth and consider creating sub-suite targets (check-opencl-simple-AMDAPP_SDK)
2. Consider removing -simple from the targets, maybe use -heavy in some future targets if needed.
3. Add examples of more complex tests with common utilities.
4. Add example with output filtering (to exclude unnecessary or changing part of the output).
5. Support specific GPU target
6. Do experiments with other OpenCL implementations