test: rename integrate-ava package to integrate-vitest (#663)

Follow-up to #659. That PR migrated the `integrate-ava` package from ava
to vitest, leaving the package name/directory a misnomer.

Rename `packages/integrate-ava` → `packages/integrate-vitest` so it's
named for what it integrates, consistent with the sibling
`integrate-module` / `integrate-module-bundler` packages.

- `git mv` of the directory (all files are pure renames; only the `name`
field in `package.json` changes).
- `pnpm-lock.yaml`: the importer key is renamed and moved to its sorted
position — no dependency changes (`pnpm install --frozen-lockfile`
passes).
- The root `--filter=integrate*` test glob still matches, and the specs'
relative paths (`../../cli/dist`, `../core`) are unchanged (same
directory depth).

`pnpm test` is green — `integrate-vitest` 9/9, plus `integrate-module`
21 and `integrate-module-bundler` 22.

Author: Boshen