TST: Add explicit npm test case

Author: kernc

tests/test-npm.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+set -eu
+
+. "${0%/*}/_init.sh"
+
+sandbox-run npm install dotenv
+
+# Assert files exist
+test -d "$tmpdir"/node_modules/dotenv*
+test -d "$tmpdir"/.sandbox*/.npm