testutils: Add assert_dict_has

This is a convenient way for tests to assert that some nested dicts
(like a parsed json) has a particular key/value somewhere in it.

For example:
  assert_dict_has(config, "toplevel.subitem.key", True)

osbuild/testutil/__init__.py

@@ -6,3 +6,10 @@
 
 def has_executable(executable: str) -> bool:
     return shutil.which(executable) is not None
+
+
+def assert_dict_has(v, keys, expected_value):
+    for key in keys.split("."):
+        assert key in v
+        v = v[key]
+    assert v == expected_value