tests: use teardown method

Avoids a side effect when the first test class fails, leaving the
system in an unclean state for the next test class.

testsuite/python/stress.py

@@ -90,6 +90,9 @@ def stress_nonbonded_intra(particle_pairs):
 @utx.skipIfMissingFeatures(['LENNARD_JONES'])
 class Stress(ut.TestCase):
 
+    def tearDown(self):
+        system.part.clear()
+
     def test(self):
         # system parameters
         system.box_l = 3 * [10.0]
@@ -244,12 +247,13 @@ def test(self):
             sim_pressure["total"],
             delta=tol)
 
-        system.part.clear()
-
 
 @utx.skipIfMissingFeatures(['EXTERNAL_FORCES'])
 class StressFENE(ut.TestCase):
 
+    def tearDown(self):
+        system.part.clear()
+
     def get_anal_stress_fene(self, pos_1, pos_2, k, d_r_max, r_0):
         stress = np.zeros([3, 3])
         vec_r = pos_1 - pos_2
@@ -321,8 +325,6 @@ def test_fene(self):
             sim_pressure["total"],
             delta=tol)
 
-        system.part.clear()
-
 
 if __name__ == "__main__":
     ut.main()