selftests: tc-testing: preserve list order when removing duplicates

Using set() removes duplicates but breaks ordering.
Test output should be deterministic, so replace with dict.fromkeys().

Signed-off-by: Naveen Anandhan <mr.navi8680@gmail.com>
Link: https://patch.msgid.link/20260222095536.17371-1-mr.navi8680@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/tc-testing/tdc_helper.py

@@ -16,9 +16,9 @@ def get_categorized_testlist(alltests, ucat):
 
 
 def get_unique_item(lst):
-    """ For a list, return a list of the unique items in the list. """
+    """Return unique items while preserving original order."""
     if len(lst) > 1:
-        return list(set(lst))
+        return list(dict.fromkeys(lst))
     else:
         return lst