selftests: net: print full exception on failure

Instead of a summary line print the full exception.
This makes debugging Python tests much easier.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240412141436.828666-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/lib/py/ksft.py

@@ -2,6 +2,7 @@
 
 import builtins
 import inspect
+import traceback
 from .consts import KSFT_MAIN_NAME
 
 KSFT_RESULT = None
@@ -85,7 +86,8 @@ def ksft_run(cases, args=()):
             totals['xfail'] += 1
             continue
         except Exception as e:
-            for line in str(e).split('\n'):
+            tb = traceback.format_exc()
+            for line in tb.strip().split('\n'):
                 ksft_pr("Exception|", line)
             ktap_result(False, cnt, case)
             totals['fail'] += 1