Shorten internal/vaultik tests under the race detector (closes #416)

All SQLite work in the package serializes on one allocator lock under
-race, so its run time follows the total work. Tests that need one
backup to a file:// destination now copy a shared backup made once.
The sweeper and locality fixtures are smaller, and the list truncation
test encodes one manifest instead of 1001, which cut the part of the
run that is not parallel from about 11 to 3 seconds.

Model: opus-5-5

TODO.md

@@ -24,6 +24,12 @@ owner's; a `v*` tag runs the release build in
 
 # Completed Steps
 
+- 2026-11-10: Shortened the tests of `internal/vaultik` under the race
+  detector, which on a busy host ran past the 90-second test timeout
+  ([issue #416](https://git.eeqj.de/sneak/vaultik/issues/416)). Tests
+  that need one backup to a file:// destination now copy a shared one,
+  and the largest fixtures are smaller.
+
 - 2026-11-10: Printed the status and progress lines of `remote info`
   and `snapshot verify` through `internal/ui`, with its marker and
   color, as the README describes