tests: add unit tests for BGP ibuf_scratch/ibuf_work optimization

Add test_ibuf_work.c with tests covering the in-place parsing
from ibuf_scratch and ibuf_work buffer management:

- basic parsing: length extraction, marker validation, header/message
  detection, buffer sizes
- parse_buffer: complete/partial packets, multiple messages
- scratch/work interaction: split messages across reads
- ibuf_work reuse: partial->complete->partial without reallocation
- EAGAIN handling: symmetric flow with no new data
- bgp_read: copy partial from ibuf_work to ibuf_scratch before read
- fatal error cleanup
- near-full buffer handling
- memory tracking to verify allocation only on partial packets

Signed-off-by: Enke Chen <enchen@paloaltonetworks.com>

Author: enkechen-panw

tests/bgpd/subdir.am

@@ -79,3 +79,12 @@ tests_bgpd_test_peer_attr_CPPFLAGS = $(TESTS_CPPFLAGS)
 tests_bgpd_test_peer_attr_LDADD = $(BGP_TEST_LDADD)
 tests_bgpd_test_peer_attr_SOURCES = tests/bgpd/test_peer_attr.c
 EXTRA_DIST += tests/bgpd/test_peer_attr.py
+
+
+if BGPD
+check_PROGRAMS += tests/bgpd/test_ibuf_work
+endif
+tests_bgpd_test_ibuf_work_CFLAGS = $(TESTS_CFLAGS)
+tests_bgpd_test_ibuf_work_CPPFLAGS = $(TESTS_CPPFLAGS)
+tests_bgpd_test_ibuf_work_LDADD = $(BGP_TEST_LDADD)
+tests_bgpd_test_ibuf_work_SOURCES = tests/bgpd/test_ibuf_work.c