Make mapreduce tests not print garbage

This patch removes the output looking like this when running the tests:

    reporter:counter:word,iljlk,1
    reporter:counter:word,jlk,1
    reporter:counter:word,jlk,1
    reporter:counter:word,jk,1
    reporter:counter:word,jkl,1
    reporter:counter:word,jlk,1
    reporter:counter:word,jlkj,1
    reporter:counter:word,j,1
    reporter:counter:word,ioj,1
    reporter:counter:word,ioj,1
    reporter:counter:word,kuh,1
    reporter:counter:word,kjh,1
Mute test output 'error: No task XYZ.'

You would previously see:

        test_non_existent_class (test.cmdline_test.CmdlineTest) ... nosetests: error: No task XYZ.

It's pretty annoying to ctrl+f for "error" and get false positives.
Mute testcase test_bin_luigi

Previously you saw:

    test_bin_luigi (test.cmdline_test.CmdlineTest) ... DEBUG: Checking if WriteToFile(filename=/tmp/luigi-tmp-465408515) is complete
    INFO: Scheduled WriteToFile(filename=/tmp/luigi-tmp-465408515) (PENDING)
    INFO: Done scheduling tasks
    INFO: Running Worker with 1 processes
    DEBUG: Asking scheduler for work...
    DEBUG: Pending tasks: 1
    INFO: [pid 2629] Worker Worker(salt=098531595, workers=1, host=arash-spotify-T440s, username=arash, pid=2629) running   WriteToFile(filename=/tmp/luigi-tmp-465408515)
    INFO: [pid 2629] Worker Worker(salt=098531595, workers=1, host=arash-spotify-T440s, username=arash, pid=2629) done      WriteToFile(filename=/tmp/luigi-tmp-465408515)
    DEBUG: 1 running tasks, waiting for next task to finish
    DEBUG: Asking scheduler for work...
    INFO: Done
    INFO: There are no more tasks to run at this time
    INFO: Worker Worker(salt=098531595, workers=1, host=arash-spotify-T440s, username=arash, pid=2629) was stopped. Shutting down Keep-Alive thread
Merge pull request #528 from Tarrasch/clean_nosetests_outputs

[WIP] Clean nosetests outputs