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/stub/html_data.h

@@ -6,4 +6,6 @@
 #include <stdint.h>
 typedef enum mime_type_e { mime_HTML = 0, mime_SVG, mime_ICO, mime_PNG, mime_JS, mime_CSS, mime_TXT } mime_type_t;
 struct f_data { __code char *file; uint32_t start; uint16_t len; mime_type_t mime; uint8_t gzip; };
+#define FDATA_START_login_html		0x00040000UL
+#define FDATA_START_favicon_ico		0x00041000UL
 #endif