refactor(tvix/castore): add RootNode impl for BTreeMap, mv fs tests

cl/10378 did already move store/fs to castore/fs, but we kept the tests
in tvix-store, as they were populating a PathInfoService to make nodes
appear in the mount root.

Update these tests to now just insert root nodes into a BTreeMap, and
ensure we can use that as a RootNodes too.

Change-Id: Iad7d1ee4f9423eb6e3a1da33f433842c9ae0de1f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10410
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/store/src/pathinfoservice/fs/mod.rs

@@ -10,9 +10,6 @@ use tvix_castore::{blobservice::BlobService, directoryservice::DirectoryService}
 
 use super::PathInfoService;
 
-#[cfg(test)]
-mod tests;
-
 /// Helper to construct a [TvixStoreFs] from a [BlobService], [DirectoryService]
 /// and [PathInfoService].
 /// This avoids users to have to interact with the wrapper struct directly, as