diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index 5f544a3129..2453172bec 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -1239,11 +1239,11 @@ def test_user_config_path_is_default_when_env_var_is_empty(working_env):
     assert os.path.expanduser("~%s.spack" % os.sep) == spack.paths._get_user_config_path()
 
 
-def test_default_install_tree(monkeypatch):
+def test_default_install_tree(monkeypatch, default_config):
     s = spack.spec.Spec("nonexistent@x.y.z %none@a.b.c arch=foo-bar-baz")
     monkeypatch.setattr(s, "dag_hash", lambda: "abc123")
-    projection = spack.config.get("config:install_tree:projections:all", scope="defaults")
-    assert s.format(projection) == "foo-bar-baz/none-a.b.c/nonexistent-x.y.z-abc123"
+    _, _, projections = spack.store.parse_install_tree(spack.config.get("config"))
+    assert s.format(projections["all"]) == "foo-bar-baz/none-a.b.c/nonexistent-x.y.z-abc123"
 
 
 def test_local_config_can_be_disabled(working_env):
diff --git a/lib/spack/spack/test/conftest.py b/lib/spack/spack/test/conftest.py
index 514b1e9154..bfd32a8f3b 100644
--- a/lib/spack/spack/test/conftest.py
+++ b/lib/spack/spack/test/conftest.py
@@ -629,7 +629,7 @@ def platform_config():
     spack.config.add_default_platform_scope(spack.platforms.real_host().name)
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture
 def default_config():
     """Isolates the default configuration from the user configs.