Add construct_from_cl_mem test-case to test_buffer

This adds a new test-case to test_buffer which checks the
functionality of the buffer(cl_mem) constructor.
Add construct_from_cl_command_queue test-case to test_command_queue

This adds a new test-case to test_command_queue which checks the
functionality of the command_queue(cl_command_queue) constructor.
Add construct_from_cl_context test-case to test_context

This adds a new test executable for the context class and
adds a new test-case which checks the functionality of the
context(cl_context) constructor.