testutil: extract find_one_subclass_in_module() helper

A small refactor to avoid shipping this duplicated code (this
one is easy to extract/reuse).

osbuild/testutil/__init__.py

@@ -2,6 +2,7 @@
 Test related utilities
 """
 import contextlib
+import inspect
 import os
 import pathlib
 import re
@@ -10,6 +11,8 @@ import socket
 import subprocess
 import tempfile
 import textwrap
+from types import ModuleType
+from typing import Type
 
 
 def has_executable(executable: str) -> bool:
@@ -168,3 +171,18 @@ def make_fake_service_fd() -> int:
     sock = socket.socket(socket.AF_UNIX, socket.SOCK_SEQPACKET)
     fd = os.dup(sock.fileno())
     return fd
+
+
+def find_one_subclass_in_module(module: ModuleType, subclass: Type) -> object:
+    """Find the class in the given module that is a subclass of the given input
+
+    If multiple classes are found an error is raised.
+    """
+    cls = None
+    for name, memb in inspect.getmembers(
+            module,
+            predicate=lambda obj: inspect.isclass(obj) and issubclass(obj, subclass)):
+        if cls:
+            raise ValueError(f"already have {cls}, also found {name}:{memb}")
+        cls = memb
+    return cls