Add cmocka-based test program for SO_TIMESTAMPING socket option
under testing/nettest/timestamp/. Covers all five test scenarios
from the SO_TIMESTAMPING PR (apache/nuttx#20161):
1. SO_TIMESTAMPING setsockopt/getsockopt
2. SO_TIMESTAMP/SO_TIMESTAMPNS backward compatibility
3. TX timestamping + MSG_ERRQUEUE delivery
4. RX timestamp via SO_TIMESTAMP
5. TX timestamping with poll(POLLPRI) notification
Enable with CONFIG_TESTING_NET_TIMESTAMP=y (requires
CONFIG_NET_TIMESTAMP, CONFIG_NET_PKT, CONFIG_TESTING_CMOCKA).
Tested on sim:dynconns — all 5 tests pass.
Signed-off-by: wenquan1 <wenquan1@xiaomi.com>