test/syscalls: add recvfrom restart regression test

[Amaindex]

This adds regression coverage for the recvfrom restart behavior fixed by #13212.

The test covers this syscall shape on a Unix SOCK_SEQPACKET socket:

recvfrom(fd, nullptr, 0, MSG_TRUNC | MSG_PEEK, nullptr, nullptr)

Linux restarts this syscall when the blocking thread receives a signal whose handler was installed with SA_RESTART. The test installs a SIGURG handler with SA_RESTART, interrupts the blocked recvfrom, then sends one packet and expects recvfrom to return the packet length.

This syscall shape is relevant to runc's sync socket packet-length probe, which is the path mentioned in #13179.

Related to #13179.

Regression coverage for #13212.

Local verification

make test TARGETS=//test/syscalls:socket_non_stream_blocking_local_test_native \
  OPTIONS="--test_output=errors --nocache_test_results"

Result: PASS

make syscall-tests TARGETS=//test/syscalls:socket_non_stream_blocking_local_test_runsc_systrap_directfs \
  OPTIONS="--test_output=errors --nocache_test_results"

Result: PASS

[Amaindex]

Quick update: #13212 has landed, so I rebased this onto the fixed baseline and marked it ready for review.

This is now just regression coverage for the reduced recvfrom case from #13179/#13212. I re-ran the native and runsc systrap/directfs targets locally after the rebase, and both pass.

[milantracy]

thanks a lot!