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.

var/spack/repos/builtin.mock/packages/version-test-dependency-preferred/package.py

@@ -0,0 +1,16 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+class VersionTestDependencyPreferred(AutotoolsPackage):
+    """Dependency of version-test-pkg, which has a multi-valued
+    variant with two default values (a very low priority optimization
+    criterion for clingo is to maximize their number)
+    """
+    homepage = "http://www.spack.org"
+    url = "http://www.spack.org/downloads/xz-1.0.tar.gz"
+
+    version('5.2.5', sha256='5117f930900b341493827d63aa910ff5e011e0b994197c3b71c08a20228a42df')
+
+    variant('libs', default='shared,static', values=('shared', 'static'),
+            multi=True, description='Build shared libs, static libs or both')

var/spack/repos/builtin.mock/packages/version-test-pkg/package.py

@@ -0,0 +1,19 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+class VersionTestPkg(AutotoolsPackage):
+    """Mock AutotoolsPackage to check proper version
+    selection by clingo.
+    """
+    homepage = "https://www.gnu.org/software/make/"
+    url = "http://www.example.com/libtool-version-1.0.tar.gz"
+
+    version('develop', git='https://git.savannah.gnu.org/git/libtool.git',
+            branch='master', submodules=True)
+    version('2.4.6', sha256='e40b8f018c1da64edd1cc9a6fce5fa63b2e707e404e20cad91fbae337c98a5b7')
+
+    depends_on(
+        'version-test-dependency-preferred',
+        when='@develop'
+    )

var/spack/repos/builtin.mock/packages/version-test-root/package.py

@@ -0,0 +1,11 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+class VersionTestRoot(AutotoolsPackage):
+    """Uses version-test-pkg, as a build dependency"""
+    homepage = "http://www.spack.org"
+    url = "http://www.spack.org/downloads/aml-1.0.tar.gz"
+
+    version('0.1.0', sha256='cc89a8768693f1f11539378b21cdca9f0ce3fc5cb564f9b3e4154a051dcea69b')
+    depends_on('version-test-pkg', type='build')