tests/plugins/loading.py: Refactor test to not depend on yaml.dump() formatting

This test was writing the YAML with ruamel.yaml and then asserting that we
have the correct provenance, however as ruamel.yaml evolves we cannot be sure
that the dump formatting will be exactly the same (as it has in this instance).

To avoid confusion, create static copies of the project.conf so that the
provenance can be tested properly.
diff --git a/tests/plugins/loading.py b/tests/plugins/loading.py
index 7aeb242..2c2058c 100644
--- a/tests/plugins/loading.py
+++ b/tests/plugins/loading.py
@@ -679,29 +679,12 @@
 @pytest.mark.datafiles(DATA_DIR)
 @pytest.mark.parametrize(
     "plugin_type,provenance",
-    [("elements", "project.conf [line 10 column 2]"), ("sources", "project.conf [line 10 column 2]")],
+    [("elements", "project.conf [line 12 column 2]"), ("sources", "project.conf [line 12 column 2]")],
 )
 def test_junction_invalid_full_path(cli, datafiles, plugin_type, provenance):
     project = str(datafiles)
-    subproject = os.path.join(project, "subproject")
-    subsubproject = os.path.join(subproject, "subsubproject")
 
-    shutil.copytree(os.path.join(project, "plugins"), os.path.join(subsubproject, "plugins"))
-
-    # The toplevel says to search for the "notfound" plugin in the subproject
-    #
-    update_project(
-        project,
-        {
-            "plugins": [
-                {
-                    "origin": "junction",
-                    "junction": "subproject-junction.bst:pony-junction.bst",
-                    plugin_type: ["notfound"],
-                }
-            ]
-        },
-    )
+    shutil.copy(os.path.join(project, "not-found-{}.conf".format(plugin_type)), os.path.join(project, "project.conf"))
     setup_element(project, plugin_type, "notfound")
 
     result = cli.run(project=project, args=["show", "element.bst"])
diff --git a/tests/plugins/loading/not-found-elements.conf b/tests/plugins/loading/not-found-elements.conf
new file mode 100644
index 0000000..71e7ad9
--- /dev/null
+++ b/tests/plugins/loading/not-found-elements.conf
@@ -0,0 +1,15 @@
+# This project.conf gets rewritten for each plugin loading test
+name: test
+
+# Required BuildStream version
+min-version: 2.0
+
+# Subdirectory where elements are stored
+element-path: elements
+
+# Load non-existant element across junction boundaries
+plugins:
+- origin: junction
+  junction: subproject-junction.bst:pony-junction.bst
+  elements:
+  - notfound
diff --git a/tests/plugins/loading/not-found-sources.conf b/tests/plugins/loading/not-found-sources.conf
new file mode 100644
index 0000000..404a20a
--- /dev/null
+++ b/tests/plugins/loading/not-found-sources.conf
@@ -0,0 +1,15 @@
+# This project.conf gets rewritten for each plugin loading test
+name: test
+
+# Required BuildStream version
+min-version: 2.0
+
+# Subdirectory where elements are stored
+element-path: elements
+
+# Load non-existant source across junction boundaries
+plugins:
+- origin: junction
+  junction: subproject-junction.bst:pony-junction.bst
+  sources:
+  - notfound