Stop using test dependency to get list of snapshots

In general tests inherit from their test dependency and so the list of
snapshots will already be defined. This also allows extra flexibility to
create a test that does not create all snapshots created by its test
dependency. Although it cannot create additional ones currently.

reframe_tests/tests/mixins.py

@@ -45,24 +45,10 @@ class SnapshotMixin(GprMaxMixin):
 
     @run_after("setup")
     def add_snapshot_regression_checks(self):
-        has_specified_snapshots = len(self.snapshots) > 0
-        valid_test_dependency = self.test_dependency is not None and issubclass(
-            self.test_dependency, SnapshotMixin
-        )
-
         self.skip_if(
-            not valid_test_dependency and not has_specified_snapshots,
-            f"Must provide either a list of snapshots, or a test dependency that inherits from SnapshotMixin.",
+            len(self.snapshots) < 0,
+            f"Must provide a list of snapshots.",
         )
-        self.skip_if(
-            valid_test_dependency and has_specified_snapshots,
-            f"Cannot provide both a list of snapshots, and a test dependency that inherits from SnapshotMixin.",
-        )
-
-        if valid_test_dependency:
-            target = self.get_test_dependency()
-            assert isinstance(target, SnapshotMixin)
-            self.snapshots = target.snapshots
 
         for snapshot in self.snapshots:
             snapshot_file = self.build_snapshot_filepath(snapshot)
@@ -104,8 +90,6 @@ class BScanMixin(GprMaxMixin):
 
 
 class TaskfarmMixin(GprMaxMixin):
-    extra_executable_opts = ["-taskfarm"]
-
     # num_tasks = required
 
     @run_after("setup")