test/old: detect user modules for python,ruby

Set the environment variables before changing $HOME
so that python,ruby can find user modules not in $HOME.

Reference:
- 7be7ec98a2
- https://github.com/neovim/neovim/issues/10270#issuecomment-531516948

src/nvim/testdir/setup.vim

@@ -31,12 +31,6 @@ let &packpath = &rtp
 " Avoid storing shell history.
 let $HISTFILE = ""
 
-" Make sure $HOME does not get read or written.
-let $HOME = expand(getcwd() . '/XfakeHOME')
-if !isdirectory($HOME)
-  call mkdir($HOME)
-endif
-
 " Use default shell on Windows to avoid segfault, caused by TUI
 if has('win32')
   let $SHELL = ''
@@ -45,3 +39,15 @@ if has('win32')
   set shellcmdflag=/s/c shellxquote=\" shellredir=>%s\ 2>&1
   let &shellpipe = &shellredir
 endif
+
+" Detect user modules for language providers
+let $PYTHONUSERBASE = $HOME . '/.local'
+if executable('gem')
+  let $GEM_PATH = system('gem env gempath')
+endif
+
+" Make sure $HOME does not get read or written.
+let $HOME = expand(getcwd() . '/XfakeHOME')
+if !isdirectory($HOME)
+  call mkdir($HOME)
+endif