Regression test for version selection with preferences (#25602)

This commit adds a regression test for version selection
with preferences in `packages.yaml`. Before PR 25585 we
used negative weights in a minimization to select the
optimal version. This may lead to situations where a
dependency may make the version score of dependents
"better" if it is preferred in packages.yaml.

lib/spack/spack/test/concretize_preferences.py

@@ -384,3 +384,15 @@ def test_variant_not_flipped_to_pull_externals(self):
 
         assert '~external' in s['vdefault-or-external']
         assert 'externaltool' not in s
+
+    @pytest.mark.regression('25585')
+    def test_dependencies_cant_make_version_parent_score_better(self):
+        """Test that a package can't select a worse version for a
+        dependent because doing so it can pull-in a dependency
+        that makes the overall version score even or better and maybe
+        has a better score in some lower priority criteria.
+        """
+        s = Spec('version-test-root').concretized()
+
+        assert s.satisfies('^version-test-pkg@2.4.6')
+        assert 'version-test-dependency-preferred' not in s

lib/spack/spack/test/data/config/packages.yaml

@@ -47,3 +47,5 @@ packages:
     externals:
       - spec: external-non-default-variant@3.8.7~foo~bar
         prefix: /usr
+  version-test-dependency-preferred:
+    version: ['5.2.5']