patch 8.1.1546: in some tests 'tags' is set but not restored

Problem:    In some tests 'tags' is set but not restored. (Daniel Hahler)
Solution:   Restore 'tags'. (closes #4535)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 9d3065d..e8eaef0 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -237,6 +237,7 @@
 
   call feedkeys(":set tags=./\\\\ dif\<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_equal('"set tags=./\\ diff diffexpr diffopt', @:)
+  set tags&
 endfunc
 
 func Test_set_errors()