Fix empty install prefix post-install sanity check (#30983)

var/spack/repos/builtin.mock/packages/failing-empty-install/package.py

@@ -0,0 +1,16 @@
+# Copyright 2013-2022 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)
+from spack.package import *
+
+
+class FailingEmptyInstall(Package):
+    """This package installs nothing, install should fail."""
+    homepage = "http://www.example.com/trivial_install"
+    url      = "http://www.unit-test-should-replace-this-url/trivial_install-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    def install(self, spec, prefix):
+        pass