Assert kept content in the update-error test, not just size (#176 follow-up)

44_local-update-errormask checked only keep.dat's size (>= 1024). The good
body carries a distinctive KEEP marker that nothing asserted, so a same-size
wrong-content overwrite would still pass. Also assert the content matches
^KEEP and lacks the 403 error body, so the test discriminates the fix from
an equal-size clobber the size check alone misses.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

tests/44_local-update-errormask.test

@@ -8,4 +8,6 @@ set -euo pipefail
 bash "$top_srcdir/tests/local-crawl.sh" --rerun \
     --found 'errmask/keep.dat' \
     --file-min-bytes 'errmask/keep.dat' 1024 \
+    --file-matches 'errmask/keep.dat' '^KEEP' \
+    --file-not-matches 'errmask/keep.dat' 'error 403' \
     httrack 'BASEURL/errmask/index.html'