Include unit_tests in Manifest.get_resouce_fqns

Because `unit_tests` weren't being included in calls to `Manifest.get_resource.fqns`,
it always appeared to `_warn_for_unused_resource_config_paths` that there were no
unit tests in the manifest. Because of this `_warn_for_unused_resource_config_paths` thought
that _any_ `unit_test` config in `dbt_project.yaml` was unused.

.changes/unreleased/Fixes-20240613-183117.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body: DOn't warn on `unit_test` config paths that are properly used
+time: 2024-06-13T18:31:17.486497-07:00
+custom:
+  Author: QMalcolm
+  Issue: "10311"

core/dbt/contracts/graph/manifest.py

@@ -988,6 +988,7 @@ def get_resource_fqns(self) -> Mapping[str, PathSet]:
             self.metrics.values(),
             self.semantic_models.values(),
             self.saved_queries.values(),
+            self.unit_tests.values(),
         )
         for resource in all_resources:
             resource_type_plural = resource.resource_type.pluralize()