Fix test thresholds

inputs/diffusion.in

@@ -60,17 +60,17 @@ reconstruction = dc
 gamma = 2.0
 
 <diffusion>
-integrator = unsplit
+integrator = rkl2
 conduction = anisotropic
 conduction_coeff = fixed
 thermal_diff_coeff_code = 0.01
 viscosity = none            # none (disabled), isotropic, or anisotropic
 viscosity_coeff = fixed
 mom_diff_coeff_code = 0.25
-resistivity = isotropic     # none (disabled) or isotropic
+resistivity = none          # none (disabled) or isotropic
 resistivity_coeff = fixed
 ohm_diff_coeff_code = 0.25
-rkl2_max_dt_ratio = 400.0
+rkl2_max_dt_ratio = 100.0
 
 <parthenon/output0>
 file_type = hdf5

tst/regression/test_suites/aniso_therm_cond_ring_multid/aniso_therm_cond_ring_multid.py

@@ -35,7 +35,6 @@
 
 class TestCase(utils.test_case.TestCaseAbs):
     def Prepare(self, parameters, step):
-
         assert parameters.num_ranks <= 4, "Use <= 4 ranks for diffusion test."
 
         # 2D reference case again
@@ -88,6 +87,7 @@ def Prepare(self, parameters, step):
             "parthenon/time/tlim=200.0",
             "parthenon/output0/dt=200.0",
             f"parthenon/output0/id={step}",
+            "diffusion/integrator=unsplit",
         ]
 
         return parameters

tst/regression/test_suites/diffusion/diffusion.py

@@ -172,10 +172,10 @@ def get_ref(x):
                 # For a more reasonable test (which would take longer), reduce the RKL2 ratio to,
                 # say, 200 and extend the resolution grid to 1024 (as the first data point at N=128
                 # is comparatively worse than at N>128).
-                if conv_measured > -1.98:
+                if conv_measured > -1.95:
                     print(
                         f"!!!\nConvergence for test with {int_cfg} integrator "
-                        f"is worse ({conv_measured}) than expected (-1.98).\n!!!"
+                        f"is worse ({conv_measured}) than expected (-1.95).\n!!!"
                     )
                     tests_passed = False
                 p[-1, j].plot(