Before: make -sr -j2 UITest_writer_tests UITest_writer_tests2 -> 5m9,863s
After: make -sr -j4 UITest_writer_tests UITest_writer_tests2 UITest_writer_tests3 UITest_writer_tests4 -> 2m21,734s

Again just assuming that each .py file means the same amount of CPU cost
(which may not be true exactly).

Change-Id: I9c4144c40eb1116d4b999bbba68e2fbef440af7b
Reviewed-on: https://gerrit.libreoffice.org/56880Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins