Add a more interesting test

In this one, we don't just output an existing derivation as is, but
modify it first.

tests/dyn-drv/recursive-mod-json.sh

@@ -0,0 +1,25 @@
+source common.sh
+
+# FIXME
+if [[ $(uname) != Linux ]]; then skipTest "Not running Linux"; fi
+
+enableFeatures 'recursive-nix'
+restartDaemon
+
+clearStore
+
+rm -f $TEST_ROOT/result
+
+EXTRA_PATH=$(dirname $(type -p nix)):$(dirname $(type -p jq))
+export EXTRA_PATH
+
+# Will produce a drv
+metaDrv=$(nix-instantiate ./recursive-mod-json.nix)
+
+# computed "dynamic" derivation
+drv=$(nix-store -r $metaDrv)
+
+# build that dyn drv
+res=$(nix-store -r $drv)
+
+grep 'I am alive!' $res/hello