Set/Override config.default_funder_id for tests

The 'User creates a new Plan' test will not pass unless an org exists within the db such that  `org.id == Rails.application.config.default_funder_id`. However, we have `config.default_funder_id = Rails.application.secrets.default_funder_id` within `config/application.rb`. This secret is not read while executing the tests via our GitHub actions. Thus, we are overriding/setting that value within the test itself.

spec/features/plans_spec.rb

@@ -10,7 +10,8 @@
     @org = create(:org)
     @research_org = create(:org, :organisation, :research_institute,
                            name: 'Test Research Org', templates: 1)
-    @funding_org  = create(:org, :funder, templates: 1, name: Rails.application.config.default_funder_name)
+    @funding_org  = create(:org, :funder, templates: 1)
+    Rails.application.config.default_funder_id = @funding_org.id
     @template     = create(:template, org: @org)
     @user         = create(:user, org: @org)
     sign_in(@user)