chore(dev-deps): bump @salesforce/cli-plugins-testkit

Bumps [@salesforce/cli-plugins-testkit](https://github.com/salesforcecli/cli-plugins-testkit) from 5.3.16 to 5.3.21.
- [Release notes](https://github.com/salesforcecli/cli-plugins-testkit/releases)
- [Changelog](https://github.com/salesforcecli/cli-plugins-testkit/blob/main/CHANGELOG.md)
- [Commits](salesforcecli/cli-plugins-testkit@5.3.16...5.3.21)

---
updated-dependencies:
- dependency-name: "@salesforce/cli-plugins-testkit"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>