Fix #1600: add end-2-end test of hix project initialization and flakes development shell

Author: yvan-sraka

test/tests.sh

@@ -140,4 +140,13 @@ nix build $NIX_BUILD_ARGS --no-link \
     extra-hackage.run.project.hsPkgs.external-package-user.components.exes.external-package-user
 echo >& 2
 
+printf "*** End-2-end test of hix project initialization and flakes development shell ...\n" >& 2
+HASKELL_NIX=$(pwd)/..
+cd $(mktemp -d)
+nix-shell -p cabal-install --run "cabal update; cabal unpack hello"
+cd hello-*
+nix run $HASKELL_NIX#hix -- init
+nix develop --override-input haskellNix $HASKELL_NIX -c "cabal build"
+echo >& 2
+
 printf "\n*** Finished successfully\n" >& 2