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/Makefile

@@ -26,7 +26,8 @@ BUILDDIR = build
 # One binary per subsystem under test.
 TESTS = $(BUILDDIR)/test_cmd_editor \
         $(BUILDDIR)/test_port_tables \
-        $(BUILDDIR)/test_page_json
+        $(BUILDDIR)/test_page_json \
+        $(BUILDDIR)/test_httpd_tx
 
 # The table tests link the register mock and one environment file; stub/
 # stands in for the two headers a firmware build generates.
@@ -49,6 +50,11 @@ $(BUILDDIR)/test_port_tables: test_port_tables.c $(TABLE_ENV) hw_mock.h | $(BUIL
 $(BUILDDIR)/test_page_json: test_page_json.c $(TABLE_ENV) hw_mock.h | $(BUILDDIR)
 	$(CC) $(CFLAGS) $(filter %.c,$^) -o $@ $(LDFLAGS)
 
+# httpd/httpd.c and uip/uip.c are compiled UNMODIFIED from the firmware tree;
+# this test carries the environment below them itself.
+$(BUILDDIR)/test_httpd_tx: test_httpd_tx.c $(SRCROOT)/httpd/httpd.c $(SRCROOT)/uip/uip.c | $(BUILDDIR)
+	$(CC) $(CFLAGS) $(filter %.c,$^) -o $@ $(LDFLAGS)
+
 run: $(TESTS)
 	@fail=0; for t in $(TESTS); do \
 		echo "=== $$t ==="; \