Split tests some more

Good for parallelism and easier reading.

tests/overlay-local-store/redundant-add-inner.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+
+set -eu -o pipefail
+
+set -x
+
+source common.sh
+
+storeDirs
+
+initLowerStore
+
+mountOverlayfs
+
+### Do a redundant add
+
+# upper layer should not have it
+expect 1 stat $(toRealPath "$storeBTop/nix/store" "$path")
+
+path=$(nix-store --store "$storeB" --add ../dummy)
+
+# lower store should have it from before
+stat $(toRealPath "$storeA/nix/store" "$path")
+
+# upper layer should still not have it (no redundant copy)
+expect 1 stat $(toRealPath "$storeB/nix/store" "$path")