chore: fix pruner and reify tests for optional peer deps

[liamcmitchell]

While investigating CI failure in #8537, I found unrelated test failures when running locally on windows.

prune with lockfile with implicit optional peer dependencies fixed by adding files to fixture node_modules, avoiding network, also reduced package lock to minimum and removed unhelpful snapshots

move aside symlink clutter fixed by creating problematic symlink in t.testdir() instead of in reifyPackages hook

[G-Rath]

awesome thanks for helping improve this! just one note:

also reduced package lock to minimum and removed unhelpful snapshots

While I agree we don't need all the packages, I think it would be best to have retained a few of the child dependencies because that would show direct-dependencies-of-optional-peer-dependencies are marked as such, and thus are also successfully pruned (ideally it'd also be good to have a test going the other way, with a dependency that is both used by e.g. babel-plugin-macros and a direct dependency, and thus should not be pruned)