selftests: net: add more info to error in bpf_offload

bpf_offload caught a spurious warning in TC recently, but the error
message did not provide enough information to know what the problem
is:

FAIL: Found 'netdevsim' in command output, leaky extack?

Add the extack to the output:

FAIL: Unexpected command output, leaky extack? ('netdevsim', 'Warning: Filter with specified priority/protocol not found.')

Acked-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/bpf_offload.py

···
     check_extack(output, "netdevsim: " + reference, args)
 
 def check_no_extack(res, needle):
-    fail((res[1] + res[2]).count(needle) or (res[1] + res[2]).count("Warning:"),
-         "Found '%s' in command output, leaky extack?" % (needle))
+    haystack = (res[1] + res[2]).strip()
+    fail(haystack.count(needle) or haystack.count("Warning:"),
+         "Unexpected command output, leaky extack? ('%s', '%s')" % (needle, haystack))
 
 def check_verifier_log(output, reference):
     lines = output.split("\n")