selftests/bpf: Test FIONREAD on a sockmap socket without a verdict program

selftests/bpf: Test FIONREAD on a sockmap socket without a verdict program

Add a test validating that FIONREAD on a TCP socket in a sockmap
without a verdict program reports data pending in sk_receive_queue.

Signed-off-by: Mattia Meleleo <mattia.meleleo@coralogix.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20260708-fionread-no-verdict-v3-2-b4ee31b3af53@coralogix.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>