tests: include load_tests for python -m unittest discover

Since we switched to running tests with python -m unittest discover,
doctests were not longer ran.
2 jobs for feat/linecache in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:44:25

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:54:16