reset rmg database to None in testGenerateReactions
reset rmg database to None in testGenerateReactions

avoids problems of loading it when it already exists.

nyee/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py