libexpr-tests: Add unit tests for broken readDir /. for pure eval

A very unfortunate interaction of current filtering with pure eval is
that the following actually leads to `lib.a = {}`. This just adds a unit
test for this broken behavior. This is really good to be done as a unit test
via the in-memory store.

{
  outputs =
    { ... }:
    {
      lib.a = builtins.readDir /.;
    };
}

src/libstore-test-support/include/nix/store/tests/libstore.hh

@@ -19,14 +19,13 @@ public:
     }
 
 protected:
+    LibStoreTest(ref<Store> store)
+        : store(std::move(store))
+    {
+    }
+
     LibStoreTest()
-        : store(openStore({
-              .variant =
-                  StoreReference::Specified{
-                      .scheme = "dummy",
-                  },
-              .params = {},
-          }))
+        : LibStoreTest(openStore("dummy://"))
     {
     }