Add test for store optimise path deduplication.

tests/overlay-local-store/local.mk

@@ -4,6 +4,7 @@ overlay-local-store-tests := \
   $(d)/build.sh \
   $(d)/bad-uris.sh \
   $(d)/add-lower.sh \
-  $(d)/verify.sh
+  $(d)/verify.sh \
+  $(d)/optimise.sh
 
 install-tests-groups += overlay-local-store

tests/overlay-local-store/optimise-inner.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+set -eu -o pipefail
+
+set -x
+
+source common.sh
+
+# Avoid store dir being inside sandbox build-dir
+unset NIX_STORE_DIR
+unset NIX_STATE_DIR
+
+storeDirs
+
+initLowerStore
+
+mountOverlayfs
+
+nix-store --store "$storeB" --optimise

tests/overlay-local-store/optimise.sh

@@ -0,0 +1,5 @@
+source common.sh
+
+requireEnvironment
+setupConfig
+execUnshare ./optimise-inner.sh