So that e.g. unit tests can be easily run with OpenCL forced. This forces
even single cells to be evaluated using the forced method (many correctness
tests are just a single cell which normally would not be used for grouped
calculation).

Change-Id: If5c6e77a6e0d8696d5416d760cf5e47b8acf3d27
Reviewed-on: https://gerrit.libreoffice.org/63188
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>