oldtests: newstyle: win: reset shell to cmd.exe

Unset $SHELL so that child nvim use cmd.exe as default shell.

Unset $TERM so that child nvim don't segfault with negative exit code.
sh/bash use TERM=cygwin by default if it is unset.
mintty sets TERM to xterm.

src/nvim/testdir/runtest.vim

@@ -61,7 +61,7 @@ set nomore
 lang mess C
 
 " Always use forward slashes.
-set shellslash
+" set shellslash
 
 " Prepare for calling test_garbagecollect_now().
 let v:testing = 1

src/nvim/testdir/setup.vim

@@ -17,3 +17,11 @@ let &packpath = &rtp
 
 " Make sure $HOME does not get read or written.
 let $HOME = '/does/not/exist'
+
+" Use default shell on Windows to avoid segfault, caused by TUI
+if has('win32')
+  let $SHELL = ''
+  let $TERM = ''
+  let &shell = empty($COMSPEC) ? exepath('cmd.exe') : $COMSPEC
+  set shellcmdflag=/s/c shellxquote=\" shellredir=>%s\ 2>&1
+endif