[Spec-tests] Add tests for abstract classes

compiler/tests-spec/testData/diagnostics/linked/declarations/classifier-declaration/class-declaration/abstract-classes/testsMap.json

@@ -72,7 +72,7 @@
         },
         {
           "specVersion": "0.1-213",
-          "casesNumber": 2,
+          "casesNumber": 1,
           "description": "Abstract classes may contain one or more abstract members, which should be implemented in a subtype of this abstract class",
           "unexpectedBehaviour": false
         },