Add small unit test for writing old style adjacency list.
Add small unit test for writing old style adjacency list.

chatelak/RMG-Py,nyee/RMG-Py,comocheng/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,enochd/RMG-Py,pierrelb/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py