Feature/3997 profiles test selection flag (#4270)

* Address 3997. Test selection flag can be in profile.yml.

* Per Jerco's 4104 PR unresolved comments, unify i.s. predicate and add env var.

* Couple of flake8 touchups.

* Classier error handling using enum semantics.

* Cherry-pick in part of Gerda's commit to hopefully avoid a future merge conflict.

* Add 3997 to changelog.

Co-authored-by: Mila Page <versusfacit@users.noreply.github.com>

automatic commit by git-black, original commits:
  39f350f
  5d1b104

core/dbt/graph/selector.py

@@ -93,8 +93,7 @@ def get_nodes_from_criteria(
 
         neighbors = self.collect_specified_neighbors(spec, collected)
         direct_nodes, indirect_nodes = self.expand_selection(
-            selected=(collected | neighbors),
-            indirect_selection=spec.indirect_selection
+            selected=(collected | neighbors), indirect_selection=spec.indirect_selection
         )
         return direct_nodes, indirect_nodes