test: Move make_fake_input_tree to testutil

This is useful for other stage tests, move it and add a test.

osbuild/testutil/__init__.py

@@ -1,6 +1,7 @@
 """
 Test related utilities
 """
+import os
 import shutil
 
 
@@ -13,3 +14,20 @@ def assert_dict_has(v, keys, expected_value):
         assert key in v
         v = v[key]
     assert v == expected_value
+
+
+def make_fake_input_tree(tmpdir, fake_content: dict) -> str:
+    """Create a directory tree of files with content.
+
+    Call it with:
+        {"filename": "content", "otherfile": "content"}
+
+    filename paths will have their parents created as needed, under tmpdir.
+    """
+    basedir = os.path.join(tmpdir, "tree")
+    for path, content in fake_content.items():
+        dirp, name = os.path.split(os.path.join(basedir, path.lstrip("/")))
+        os.makedirs(dirp, exist_ok=True)
+        with open(os.path.join(dirp, name), "w", encoding="utf-8") as fp:
+            fp.write(content)
+    return basedir