add test for loading and unloading ICDs (cl_khr_icd_unloadable)

[bashbaug]

This PR adds a CTS test for the cl_khr_icd_unloadable extension, see:

• OpenCL ICD Loader Deinitialization Support OpenCL-Docs#1350
• Added an OpenCL deinitialization test and plugin infrastructure. OpenCL-ICD-Loader#268

The CTS test follows a similar methodology as the ICD loader unit test:

1. The test is primarily implemented in a dynamic library "plugin".
   • The "plugin" links with the harness (and hence the OpenCL ICD loader) and makes OpenCL API calls.
   • The "plugin" currently runs a single test, which creates a kernel, executes it, checks the results, and cleans up.
2. The test executable is responsible for dynamically loading the "plugin" and invoking it to execute the test.
   • The test executable does NOT link with the harness or the OpenCL ICD loader and does not directly make any OpenCL API calls.
   • If the test executable is able to load the plugin, run the test, and unload the plugin N times (where N is currently 5), the overall test is considered to pass, and the test executable returns "success". Otherwise, the test executable returns "failure".

[bashbaug]

For now, I am mostly interested in the high-level approach. Is this something we would consider appropriate for a CTS test? Is there a lighter-weight way to test these changes?

Of course, if the high-level approach looks OK, I would be interested in detailed comments also. Thanks!