refactor(tests): allow to extend the new base set of attrs

We start at 100 so we can make the base set larger if needed. (It might need to
grow/shrink as a result of adopting the new default color scheme as the
default for tests)

Usage best illustrataded by example.

Improving the workflow for making new tests with `screen:snapshot_util()` will
be a follow up.

1 files changed, 11 insertions, 0 deletions

diff --git a/test/functional/ui/screen.lua b/test/functional/ui/screen.lua
index ee9334a9fe..ab535afbf4 100644
--- a/test/functional/ui/screen.lua
+++ b/test/functional/ui/screen.lua
@@ -255,6 +255,17 @@ function Screen:set_default_attr_ids(attr_ids)
   self._default_attr_ids = attr_ids
 end
 
+function Screen:add_extra_attr_ids(extra_attr_ids)
+  local attr_ids = vim.deepcopy(Screen._global_default_attr_ids)
+  for id, attr in pairs(extra_attr_ids) do
+    if type(id) == 'number' and id < 100 then
+      error('extra attr ids should be at least 100 or be strings')
+    end
+    attr_ids[id] = attr
+  end
+  self._default_attr_ids = attr_ids
+end
+
 function Screen:get_default_attr_ids()
   return deepcopy(self._default_attr_ids)
 end