[Test] Fix negative_A_factor to use return value

The changes from Cantera#803 regarding explicitly specifying the converted
output file didn't get included when this test was added in Cantera#822.

interfaces/cython/cantera/test/test_convert.py

@@ -265,8 +265,8 @@ def test_negative_order_permissive(self):
                            tol=2e-7)
 
     def test_negative_A_factor(self):
-        self.convert('negative-rate.inp', thermo='dummy-thermo.dat')
-        gas = ct.Solution('negative-rate.cti')  # Validate the mechanism
+        output = self.convert('negative-rate.inp', thermo='dummy-thermo.dat')
+        gas = ct.Solution(output)  # Validate the mechanism
         self.assertLess(gas.reaction(4).rate.pre_exponential_factor, 0)
         self.assertLess(gas.reaction(1).rate.pre_exponential_factor, 0)
         self.assertLess(gas.reaction(2).rate.pre_exponential_factor, 0)