Unit test discovery

[sfinkens]

Unittests starting with test__ are not discovered by python setup.py test:

...pygac/pygac/tests> grep test__ *.py

test_klm.py:    def test__validate_header(self):
test_pod.py:    def test__validate_header(self):
test_pod.py:    def test__get_calibrated_channels_uniform_shape(self, get_channels):
test_pod.py:    def test__adjust_clock_drift(self, avhrr_gac, get_tle_lines,
test_reader.py:    def test__read_scanlines(self):
test_reader.py:    def test__validate_header(self):
test_reader.py:    def test__correct_data_set_name(self):
test_reader.py:    def test__get_calibrated_channels_uniform_shape(self, get_channels):

Some of them are failing.