test: bench for the httpd transmit path

Drives the unmodified httpd.c and uip.c through a handshake and a GET
from a client that moves its receive window, and compares the bytes that
reach the wire with the file they are meant to carry. The served file
holds a position-dependent pattern, so a repeat or a gap is reported at
its offset rather than as content that differs somewhere.

The first scenario holds the window still and must pass: it is the check
that the bench itself is sound, before the two moving-window scenarios
are believed.

test/README.md

@@ -47,6 +47,7 @@ fires — so it drops straight into CI.
 | `test_cmd_editor` | `cmd_editor.c` | **C4** — full-line hang + `cmd_buffer` 1-byte overflow; basic entry & backspace regressions |
 | `test_port_tables` | `rtl837x_port.c` | VLAN entry layout and round trip, PVID register sharing, static multicast and management entries, per-port flush, trunk membership and hash seed |
 | `test_page_json` | `httpd/page_impl.c` + `rtl837x_port.c` | `/vlan.json`, `/vlanlist`, `/l2.json` (walk, wrap marker, paging inside `outbuf`), 64-bit counters in `/status.json` and `/counters.json` |
+| `test_httpd_tx` | `httpd/httpd.c` + `uip/uip.c` | a GET of a static file against a client that moves its receive window: ACK accounting, continuation chunks out of flash, retransmission |
 
 ## Adding a test for another module
 1. Write `test_<module>.c` with `main()` driving the module's entry points and