selftests: net: print report check location in python tests

Developing Python tests is a bit annoying because when test fails
we only print the fail message and no info about which exact check
led to it. Print the location (the first line of this example is new):

  # At /root/ksft-net-drv/./net/nl_netdev.py line 38:
  # Check failed 0 != 10
  not ok 3 nl_netdev.page_pool_check
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240412141436.828666-4-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

 # SPDX-License-Identifier: GPL-2.0
 
 import builtins
+import inspect
 from .consts import KSFT_MAIN_NAME
 
 KSFT_RESULT = None
@@ -18,32 +19,34 @@ def ksft_pr(*objs, **kwargs):
     print("#", *objs, **kwargs)
 
 
+def _fail(*args):
+    global KSFT_RESULT
+    KSFT_RESULT = False
+
+    frame = inspect.stack()[2]
+    ksft_pr("At " + frame.filename + " line " + str(frame.lineno) + ":")
+    ksft_pr(*args)
+
+
 def ksft_eq(a, b, comment=""):
     global KSFT_RESULT
     if a != b:
-        KSFT_RESULT = False
-        ksft_pr("Check failed", a, "!=", b, comment)
+        _fail("Check failed", a, "!=", b, comment)
 
 
 def ksft_true(a, comment=""):
-    global KSFT_RESULT
     if not a:
-        KSFT_RESULT = False
-        ksft_pr("Check failed", a, "does not eval to True", comment)
+        _fail("Check failed", a, "does not eval to True", comment)
 
 
 def ksft_in(a, b, comment=""):
-    global KSFT_RESULT
     if a not in b:
-        KSFT_RESULT = False
-        ksft_pr("Check failed", a, "not in", b, comment)
+        _fail("Check failed", a, "not in", b, comment)
 
 
 def ksft_ge(a, b, comment=""):
-    global KSFT_RESULT
     if a < b:
-        KSFT_RESULT = False
-        ksft_pr("Check failed", a, "<", b, comment)
+        _fail("Check failed", a, "<", b, comment)
 
 
 def ktap_result(ok, cnt=1, case="", comment=""):