test: add test nixosSearch

This checks that creating module docs for search.nixos.org succceeds.
Errors can happen when complex `default` values can't be evaluated
or on malformed Docbook XML in descriptions.

test/nixos-search/flake.lock

@@ -0,0 +1,44 @@
+{
+  "nodes": {
+    "nixos-search": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      },
+      "locked": {
+        "lastModified": 1638831015,
+        "narHash": "sha256-WmCsa6gSY01Yt0wminCbNV8Mn+4iU1OEF5r3YUh6SVs=",
+        "owner": "nixos",
+        "repo": "nixos-search",
+        "rev": "fa5fe2a61c36ed90506af75c19bfaf2f9d537d87",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "repo": "nixos-search",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1631118067,
+        "narHash": "sha256-tEcFvm3a6ToeBGwHdjfB2mVQwa4LZCZTQYE2LnY3ycA=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "09cd65b33c5653d7d2954fef4b9f0e718c899743",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-unstable",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixos-search": "nixos-search"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

test/nixos-search/flake.nix

@@ -0,0 +1,9 @@
+# This flake just mirrors input `nixos-search`.
+# Because `nixos-search` is a dev-only dependency, we don't add
+# it to the main flake.
+{
+  inputs.nixos-search.url = "github:nixos/nixos-search";
+  outputs = { self, nixos-search }: {
+    inherit (nixos-search) packages;
+  };
+}