task metrics testing should be more robust

[grg2rsr]

TestTaskMetrics loads fake bpod data:

class TestTaskMetrics(unittest.TestCase):
    def setUp(self):
        self.data = self.load_fake_bpod_data()
        self.wheel_gain = 4
        wheel_data = self.load_fake_wheel_data(self.data, wheel_gain=self.wheel_gain)
        self.data.update(wheel_data)

    @staticmethod
    def load_fake_bpod_data(n=5):
        """Create fake extractor output of bpodqc.load_data

        :param n: the number of trials
        :return: a dict of simulated trial data
        """

if the n is changed ( in load_fake_bpod_data(n=5), several tests start to fail.