Fix default mutable argument: filters

Otherwise filters is initialized when the function is defined and
*not* when it is called as one would expect.

Author: jmlapre

src/sst/core/testingframework/sst_unittest_support.py

@@ -1126,7 +1126,7 @@ def testing_compare_filtered_diff(
     outfile: str,
     reffile: str,
     sort: bool = False,
-    filters: Union[LineFilter, List[LineFilter]] = list(),
+    filters: Union[None, LineFilter, List[LineFilter]] = None,
     do_statistics_comparison: bool = False,
 ) -> bool:
     """Filter, optionally sort and then compare 2 files for a difference.
@@ -1145,6 +1145,9 @@ def testing_compare_filtered_diff(
 
     """
 
+    if filters is None:
+        filters = []
+
     if isinstance(filters, LineFilter):
         filters = [filters]