Refactor buffer ReadWrite and Copy tests

[MichaelRizkalla-arm]

This change refactors the following tests to use RAII to clean-up allocated resources on exit, and adds additional changes as mentioned below:

• test_arrayreadwrite
  • Allow different cl_mem_flags to be passed to the test.
• test_bufferreadwriterect:
  • Allow different cl_mem_flags to be passed to the test.
  • Customisable copy, read and write functions.
• test_buffer_copy
  • Fill the destination buffer with invalid_ptr instead of out_ptr if created with CL_MEM_(USE/COPY)_HOST_PTR.
• test_buffer_partial_copy
  • Fill the destination buffer with invalid_ptr instead of out_ptr if created with CL_MEM_(USE/COPY)_HOST_PTR.