tests: fix Test_tagfiles: use Vim's 'tags' setting

Otherwise it might fail with an existing `tags` file upwards, e.g. in
neovim's source directory (which should be rather common).

src/nvim/testdir/setup.vim

@@ -17,6 +17,8 @@ set listchars=eol:$
 set fillchars=vert:\|,fold:-
 set shortmess-=F
 set laststatus=1
+set tags=./tags,tags
+
 " Prevent Nvim log from writing to stderr.
 let $NVIM_LOG_FILE = exists($NVIM_LOG_FILE) ? $NVIM_LOG_FILE : 'Xnvim.log'
 

src/nvim/testdir/test_taglist.vim

@@ -90,7 +90,8 @@ func Test_tagfiles()
 	\           fnamemodify(tf[0], ':p:gs?\\?/?'))
   helpclose
   call assert_equal(['Xtags1', 'Xtags2'], tagfiles())
-  set tags&
+  " Nvim: defaults to "./tags;,tags", which might cause false positives.
+  set tags=./tags,tags
   call assert_equal([], tagfiles())
 
   call delete('Xtags1')