910_at_test.c and 997_at_test_ww.c build via `ww build` then read
<stem>.combined.ww as a driver side-effect; the E3 flip removes that
side-effect, so they cannot pass post-flip. Their coverage was fully
re-hosted in E2-C2 -- every axis (run-exit, -T cs/ww byte-id, the
userman/badsig/undefbody/linkfail rejects, the dup-decl family, modfn
coexist, user `fn run` coexist) has a live home in 989_septest_run,
911_attest_drop, or 989_declns_sep, several strengthened. Deleting them
before the flip keeps `make test` bisect-clean.
Also retargets a stale 'Sibling of 910_at_test' doc comment in
911_attest_record.c to the surviving gates.
Hojun-Cho
f4d809dc79