selftests: net: ksft: print more of the stack for checks

Print more stack frames and the failing line when check fails.
This helps when tests use helpers to do the checks.

Before:

  # At ./ksft/drivers/net/hw/rss_ctx.py line 92:
  # Check failed 1037698 >= 396893.0 traffic on other queues:[344612, 462380, 233020, 449174, 342298]
  not ok 8 rss_ctx.test_rss_context_queue_reconfigure

After:

  # Check| At ./ksft/drivers/net/hw/rss_ctx.py, line 387, in test_rss_context_queue_reconfigure:
  # Check|     test_rss_queue_reconfigure(cfg, main_ctx=False)
  # Check| At ./ksft/drivers/net/hw/rss_ctx.py, line 230, in test_rss_queue_reconfigure:
  # Check|     _send_traffic_check(cfg, port, ctx_ref, { 'target': (0, 3),
  # Check| At ./ksft/drivers/net/hw/rss_ctx.py, line 92, in _send_traffic_check:
  # Check|     ksft_lt(sum(cnts[i] for i in params['noise']), directed / 2,
  # Check failed 1045235 >= 405823.5 traffic on other queues (context 1)':[460068, 351995, 565970, 351579, 127270]
  not ok 8 rss_ctx.test_rss_context_queue_reconfigure

Link: https://patch.msgid.link/20240801232317.545577-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -34,8 +34,18 @@ def _fail(*args):
     global KSFT_RESULT
     KSFT_RESULT = False
 
-    frame = inspect.stack()[2]
-    ksft_pr("At " + frame.filename + " line " + str(frame.lineno) + ":")
+    stack = inspect.stack()
+    started = False
+    for frame in reversed(stack[2:]):
+        # Start printing from the test case function
+        if not started:
+            if frame.function == 'ksft_run':
+                started = True
+            continue
+
+        ksft_pr("Check| At " + frame.filename + ", line " + str(frame.lineno) +
+                ", in " + frame.function + ":")
+        ksft_pr("Check|     " + frame.code_context[0].strip())
     ksft_pr(*args)