Card-check tests: no test for the assembler's stray-figure refusal; GONE and main()'s exit tested at one point each

[realmarcin]

From the round-2 review of #212.

Where: tests/test_fleet_page.py.

• Stray figures: fleet_records() raises on markup_problems(), but no test exercises that raise for a figure outside every card (check_cards: an extra card figure is accepted, and a card with an unreadable figure is reported as missing #218). Cards with zero or two figures are also caught by assemble()'s name check, so only the stray case is truly unguarded.
• GONE: asserted only for 404. The documented rule is any 4xx except 408, 425 and 429.
• main()'s exit code: asserted only for SHRANK. The other verdicts are checked through a helper that reads FAILURES directly.

Fix:

• assert that assemble() refuses a stray figure;
• assert GONE for 403 and 410;
• assert main() returns 1 for WRONG, STALE and UNCHECKED.