Buildbot: give 20 minute per test file

It seems like at least 2 buildbots need more than 15 minutes per test file.
Example with "AMD64 Snow Leop 3.x":

    10 slowest tests:
    - test_tools: 14 min 40 sec
    - test_tokenize: 11 min 57 sec
    - test_datetime: 11 min 25 sec
    - ...