add simple calc tests for ocAdd and ocSub

Triggered by 710aeefd2 ("make opencl also use approximate add and sub"),
OpenCL doesn't actually get used to check these because it's not used
for single formulas, but it looks like handling of imprecise doubles
is not checked anywhere, so add a test.

Change-Id: I7a73af508d6ea5f141397ebf11f246cf0a622fed
Reviewed-on: https://gerrit.libreoffice.org/58068
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>