refactor(tvix/store/tests): move gen_*_service() into helper

This allows hiding to tests what exact implementation we're using, when
testing things that do something with a store, but don't care what's
used for underlying storage.

Change-Id: I7cdf60fd73c25d5050159cb31ec177db2bc2a7f1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8155
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/store/src/tests/mod.rs

@@ -1 +1,2 @@
 mod nar_renderer;
+pub mod utils;