Fixed platform issue that caused test failure on GH MacOS runner

Added in an energy check so that we test for regressions in calculating
the total energy for contexts with and without energy terms for
constraint internals

tests/convert/test_openmm.py

@@ -292,7 +292,9 @@ def test_openmm_ignore_constrained(ala_mols):
 def test_openmm_no_zero_sigmas(zero_lj_mols):
     mols = zero_lj_mols
 
-    omm = sr.convert.to(mols, "openmm", map={"constraint": "h-bonds"})
+    omm = sr.convert.to(mols, "openmm", 
+                        map={"constraint": "h-bonds",
+                             "platform": "Reference"})
 
     from openmm import XmlSerializer
 
@@ -312,15 +314,27 @@ def test_openmm_skipped_constrained_bonds(zero_lj_mols):
     omm1 = sr.convert.to(
         mols,
         "openmm",
-        map={"constraint": "h-bonds", "include_constrained_energies": True},
+        map={"constraint": "h-bonds", 
+             "include_constrained_energies": True,
+             "platform": "Reference"},
     )
 
     omm2 = sr.convert.to(
         mols,
         "openmm",
-        map={"constraint": "h-bonds", "include_constrained_energies": False},
+        map={"constraint": "h-bonds", 
+             "include_constrained_energies": False,
+             "platform": "Reference"},
     )
 
+    nrg1 = omm1.get_potential_energy().to(sr.units.kcal_per_mol)
+    nrg2 = omm2.get_potential_energy().to(sr.units.kcal_per_mol)
+
+    # Check the energies haven't changed
+    # (regression check - here are the current values)
+    assert nrg1 == pytest.approx(-447.44, 1e-3)
+    assert nrg2 == pytest.approx(-3279.87, 1e-3)
+
     from openmm import XmlSerializer
 
     xml1 = XmlSerializer.serialize(omm1.getSystem())