Before: make -sr UITest_calc_tests -> 10m48.934s

After: make -sr -j4 UITest_calc_tests UITest_calc_tests2 UITest_calc_tests3 UITest_calc_tests4 -> 5m40.113s

As a start just split based on the number of .py files in the
directories, one could improve this further by making them even more
equal based on number of lines or something.

Change-Id: I7a98a29e5d18a7600575956a7d8a0ec81e804269
Reviewed-on: https://gerrit.libreoffice.org/55174Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>