nix develop: Add a test for bash functions

tests/nix-shell.sh

@@ -101,3 +101,4 @@ source <(nix print-dev-env -f shell.nix shellDrv)
 [[ ${arr1[2]} = "3 4" ]]
 [[ ${arr2[1]} = $'\n' ]]
 [[ ${arr2[2]} = $'x\ny' ]]
+[[ $(fun) = blabla ]]

tests/shell.nix

@@ -22,6 +22,9 @@ let pkgs = rec {
     done
     declare -a arr1=(1 2 "3 4" 5)
     declare -a arr2=(x $'\n' $'x\ny')
+    fun() {
+      echo blabla
+    }
   '';
 
   stdenv = mkDerivation {