Add conftest.py

So that we clear magma cache after each pytest func (see phanrahan/magma#220).

conftest.py

@@ -0,0 +1,13 @@
+import pytest
+from magma.circuit import magma_clear_circuit_cache
+from magma import clear_cachedFunctions
+import magma.backend.coreir_ as coreir_
+
+collect_ignore = ["src"]  # pip folder that contains dependencies like magma
+
+
+@pytest.fixture(autouse=True)
+def magma_test():
+    magma_clear_circuit_cache()
+    clear_cachedFunctions()
+    coreir_.__reset_context()