Deduplicate create_image from Copy/Fill image tests

1. Remove duplicate `create_image` code that is shared between
   clFillImage and clCopyImage test directories.
2. Unify how pitch buffer's memory is deallocated; The buffer can be
   allocated with either `malloc` or `align_malloc` and the free
   function is pre-set in `pitch_buffe_data`'s member variable `free_fn`
   and used when the buffer is deallocated. With this, the change
   removes `is_aligned` conditional variable that was used to select the
   appropriate free function.

Signed-off-by: Michael Rizkalla <[email protected]>