Make subgroup specs :nested_groups

Subgroups are not supported in mySQL. I changed Namespace#root_ancestor
to return from self_and_ancestors as a bugfix.

```
  184   # Returns all the ancestors of the current namespaces
  185   def ancestors
  186     return self.class.none unless parent_id
  187
  188     Gitlab::GroupHierarchy
  189       .new(self.class.where(id: parent_id))
  190       .base_and_ancestors
  191   end
```

So it seems like on mySQL we accidentally returned the parent group :

```
ancestors = self.class.where(id: parent_id)
ancestors.reorder(nil).find_by(parent_id: nil)
```

Project#root_namespace is used only by shared_runners_limit_namespace
and all the tests for shared_runner_minutes_on_root_namespace are only
enabled on `:nested_groups`

`when :shared_runner_minutes_on_root_namespace is enabled', :nested_groups`

We very clearly state in https://docs.gitlab.com/ee/user/group/subgroups/ that `
Nested groups are only supported when you use PostgreSQL`, so I think I
will fix forward and add `:nested_groups` to the two failing feature
specs.

spec/features/ide_spec.rb

@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 describe 'IDE', :js do
-  describe 'sub-groups' do
+  describe 'sub-groups', :nested_groups do
     let(:user) { create(:user) }
     let(:group) { create(:group) }
     let(:subgroup) { create(:group, parent: group) }

spec/features/projects_spec.rb

@@ -277,7 +277,7 @@ describe 'Project' do
       end
     end
 
-    context 'for subgroups', :js do
+    context 'for subgroups', :js, :nested_groups do
       let(:group) { create(:group) }
       let(:subgroup) { create(:group, parent: group) }
       let(:project) { create(:project, :repository, group: subgroup) }