chore(tvix/store): move tests into separate file

Move them from the bottom of src/proto.rs to its own src/tests/mod.rs.

Also drop the test_ prefix, this is not golang.

Change-Id: I2e0b6b9812264f3d9721c0766936f08157fadc66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7667
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/main.rs

@@ -1,5 +1,8 @@
 mod proto;
 
+#[cfg(test)]
+mod tests;
+
 fn main() {
     println!("Hello, world!");
 }