Add a test for `nix describe-stores`

Doesn't test much, but at least ensures that the command runs properly

tests/describe-stores.sh

+source common.sh
+
+# Query an arbitrary value in `nix describe-stores --json`'s output just to
+# check that it has the right structure
+[[ $(nix --experimental-features 'nix-command flakes' describe-stores --json | jq '.["SSH Store"]["compress"]["defaultValue"]') == false ]]
+
+# Ensure that the output of `nix describe-stores` isn't empty
+[[ -n $(nix --experimental-features 'nix-command flakes' describe-stores) ]]

tests/local.mk

@@ -32,6 +32,7 @@ nix_tests = \
   post-hook.sh \
   function-trace.sh \
   recursive.sh \
+  describe-stores.sh \
   flakes.sh \
   content-addressed.sh
   # parallel.sh