validate: baseclasses: Reset Test.extra_logfiles when copying to start an iteration

Otherwise we get a big mixup with all iteration pointing to all the extra_logfiles

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8640>

subprojects/gst-devtools/validate/launcher/baseclasses.py

@@ -234,6 +234,7 @@ class Test(Loggable):
             copied_test._uuid = None
             copied_test.options = copy.copy(self.options)
             copied_test.options.logsdir = os.path.join(copied_test.options.logsdir, str(nth))
+            copied_test.extra_logfiles = set()
             os.makedirs(copied_test.options.logsdir, exist_ok=True)
 
         return copied_test