improve coverage configuration

.coveragerc

 [run]
 plugins = Cython.Coverage
-omit = */tests/*
+omit =
+     */tests/*
+     kwant/graph/dissection.py
+     kwant/_kwant_version.py
+     */__init__.py
+branch = True
+[report]
+exclude_lines =
+    skip coverage
+    @abc.abstractmethod

kwant/digest.py

@@ -72,7 +72,7 @@ def gauss(input, salt=''):
     return cos(a * TWOPI) * sqrt(-2.0 * log(1.0 - b))
 
 
-def test(n=20000):
+def test(n=20000):  # skip coverage
     """Test the generator with the dieharder suite generating n**2 samples.
 
     Executing this function may take a very long time.