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/Cargo.nix

@@ -10267,10 +10267,6 @@ rec {
           }
         ];
         devDependencies = [
-          {
-            name = "libc";
-            packageId = "libc";
-          }
           {
             name = "tempfile";
             packageId = "tempfile";