test(old): make ":h local-additions" work properly in test_help.vim

src/nvim/testdir/runtest.vim

@@ -105,7 +105,7 @@ set nomore
 lang mess C
 
 " Nvim: append runtime from build dir, which contains the generated doc/tags.
-let &runtimepath .= ','.expand($BUILD_DIR).'/runtime/'
+let &runtimepath ..= ',' .. expand($BUILD_DIR) .. '/runtime/'
 
 let s:t_bold = &t_md
 let s:t_normal = &t_me

src/nvim/testdir/test_help.vim

@@ -2,6 +2,21 @@
 
 source check.vim
 
+func SetUp()
+  let s:vimruntime = $VIMRUNTIME
+  let s:runtimepath = &runtimepath
+  " Set $VIMRUNTIME to $BUILD_DIR/runtime and remove the original $VIMRUNTIME
+  " path from &runtimepath so that ":h local-additions" won't pick up builtin
+  " help files.
+  let $VIMRUNTIME = expand($BUILD_DIR) .. '/runtime'
+  set runtimepath-=../../../runtime
+endfunc
+
+func TearDown()
+  let $VIMRUNTIME = s:vimruntime
+  let &runtimepath = s:runtimepath
+endfunc
+
 func Test_help_restore_snapshot()
   help
   set buftype=

src/nvim/testdir/test_help_tagjump.vim

@@ -1,17 +1,5 @@
 " Tests for :help! {subject}
 
-func SetUp()
-  " v:progpath is …/build/bin/nvim and we need …/build/runtime
-  " to be added to &rtp
-  let builddir = fnamemodify(exepath(v:progpath), ':h:h')
-  let s:rtp = &rtp
-  let &rtp .= printf(',%s/runtime', builddir)
-endfunc
-
-func TearDown()
-  let &rtp = s:rtp
-endfunc
-
 func Test_help_tagjump()
   help
   call assert_equal("help", &filetype)