Added test (test_group_selections_outside_atomgroup, line 516)

Test fails prior to fix to Issue #2797 (addressed in PR #5366) and passes after.
Test replicates case (in "Code to reproduce behaviour") in Issue #2797.
Expected values for test case are saved in fixture. correct_values_alphacarbons_group (line 197).

Author: DhanushkaWeerakoon

testsuite/MDAnalysisTests/analysis/test_rms.py

@@ -194,6 +194,10 @@ def correct_values_group(self):
     def correct_values_backbone_group(self):
         return [[0, 1, 0, 0, 0], [49, 50, 4.6997, 1.9154, 2.7139]]
 
+    @pytest.fixture()
+    def correct_values_alphacarbons_group(self):
+        return [[0, 1, 0, 0], [49, 50, 1.6521, 1.6371]]
+
     def test_rmsd(self, universe, correct_values, client_RMSD):
         # client_RMSD is defined in testsuite/analysis/conftest.py
         # among with other testing fixtures. During testing, it will