Skip tests that depend on the original C++

I have written a tonne of tests that compare against A&B's original C++.
Until I can distribute the `anders_briegel` package which implements
that original code, we now skip those tests if we see an ImportError.

TODO: seems a little over-zealous right now ...

tests/mock.py

@@ -4,8 +4,12 @@ Mock graphs used for testing
 
 import numpy as np
 from abp import GraphState, clifford, qi
-from anders_briegel import graphsim
 from numpy import random
+import nose
+try:
+    from anders_briegel import graphsim
+except ImportError:
+    raise nose.SkipTest("Original C++ is not available, skipping test")
 
 # We always run with A&B's CZ table when we are testing
 clifford.use_old_cz()

tests/test_against_anders_and_briegel.py

@@ -1,5 +1,4 @@
 from abp import GraphState, CircuitModel, clifford
-from anders_briegel import graphsim
 import numpy as np
 from numpy import random
 from tqdm import tqdm

tests/test_clifford.py

@@ -4,8 +4,8 @@ import itertools as it
 from abp import clifford
 from abp import build_tables
 from abp import qi
+import nose
 from nose.tools import raises
-from anders_briegel import graphsim
 
 
 def identify_pauli(m):
@@ -79,6 +79,12 @@ def test_commuters():
 
 def test_conjugation():
     """ Test that clifford.conugate() agrees with graphsim.LocCliffOp.conjugate """
+    try:
+        from anders_briegel import graphsim
+    except ImportError:
+        raise nose.SkipTest("Original C++ is not available, skipping test")
+        
+
     for operation_index, transform_index in it.product(range(4), range(24)):
         transform = graphsim.LocCliffOp(transform_index)
         operation = graphsim.LocCliffOp(operation_index)

tests/test_measurement.py

@@ -1,7 +1,6 @@
 import numpy as np
 from abp import GraphState
 from abp import qi, clifford
-from anders_briegel import graphsim
 from tqdm import tqdm
 import random
 import itertools as it

tests/test_qi.py

@@ -2,6 +2,7 @@ import numpy as np
 from abp import qi, GraphState
 from tqdm import tqdm
 import mock
+import nose
 
 DEPTH = 1000
 
@@ -126,8 +127,7 @@ def test_against_chp(n=5):
     try:
         import chp
     except ImportError:
-        print "Not testing against CHP -- not installed"
-        return
+        raise nose.SkipTest("CHP is not installed")
 
     def get_chp_state():
         """ Helper to convert CHP to CircuitModel """