refactor(tvix/store): expose fixtures, make NAR_CONTENTS const

Allow reusing CASTORE_NODE_* and NAR_CONTENTS_* from other crates.

Also, there's no need for NAR_CONTENTS_* to be Vecs of bytes, these can
just be [u8; _].

Change-Id: I435c08a9d20f6a68266d0c9a70bfc7fdb618ce42
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12915
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/store/src/pathinfoservice/cache.rs

@@ -109,8 +109,8 @@ mod test {
     use std::num::NonZeroUsize;
 
     use crate::{
+        fixtures::PATH_INFO,
         pathinfoservice::{LruPathInfoService, MemoryPathInfoService, PathInfoService},
-        tests::fixtures::PATH_INFO,
     };
 
     /// Helper function setting up an instance of a "far" and "near"