tests: add circuitbuilder build_circuit test

tests/integration/lib/test_circuitbuilder.py

+"""Integration tests for circutibuilder.py"""
+
+
+def test_build_circuit(cb):
+    # Path is empty
+    path = []
+    circuit_id, _ = cb.build_circuit(path)
+    assert not circuit_id
+    # Valid path, not valid exit
+    path = ['117A456C911114076BEB4E757AC48B16CC0CCC5F',
+            '270A861ABED22EC2B625198BCCD7B2B9DBFFC93A']
+    circuit_id, _ = cb.build_circuit(path)
+    assert not circuit_id
+    # Valid path and relays
+    # path with relay1mbyteMAB and exit1
+    path = ['117A456C911114076BEB4E757AC48B16CC0CCC5F',
+            '270A861ABED22EC2B625198BCCD7B2B9DBFFC93C']
+    circuit_id, _ = cb.build_circuit(path)
+    assert circuit_id