test: '{. -> ./run}/test_sources.py'

Move the runtime tests of source modules into the runtime-tests
subdirectory, so `make test-runtime` can pick them up.

test/run/__init__.py

@@ -0,0 +1 @@
+# The `unittest` module requires `__init__.py` to discover a directory.

test/run/test_sources.py

@@ -1,3 +1,7 @@
+#
+# Runtime Tests for Source Modules
+#
+
 import contextlib
 import ctypes
 import http.server
@@ -11,6 +15,7 @@ import unittest
 
 import osbuild.sources
 
+
 def errcheck(ret, _func, _args):
     if ret == -1:
         e = ctypes.get_errno()