refactor(tvix/castore): move src/fs/test into fuse mod r/8172

These tests only interact with the FUSE layer, and
import super::fuse to do its work.

However, this only works if the `fuse` feature is enabled, which we
don't do if we enable the `virtiofs` feature only, causing the tests
to fail:

```
❯ cargo test --no-default-features --features virtiofs
   Compiling tvix-castore v0.1.0 (/home/flokli/dev/nixos/code.tvl.fyi-submit2/tvix/castore)
error[E0432]: unresolved import `super::fuse`
  --> castore/src/fs/tests.rs:14:13
   |
14 | use super::{fuse::FuseDaemon, TvixStoreFs};
   |             ^^^^ could not find `fuse` in `super`
```

We move src/fs/tests.rs to src/fs/fuse/tests.rs
(and src/fs/fuse.rs to src/fs/fuse/mod.rs) to better structure this,
which will automatically cause both tests and code to only be built if
we have the `fuse` feature enabled.

Change-Id: I8fbbad3e4457e326bdfd171aa5c43d25d3187b5b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11715
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

0 files changed, 0 insertions, 0 deletions