Stub member access level in specs

What does this MR do and why?

This MR adds a spec helper stub_member_access_level to allow stubbing member access on factory stubs like Project or Group.

It also adds development guidelines for this helper method and converts two specs to use stub_member_access_level which improves spec runtime (see "Speed difference?" section).

Example

# Before - slow 🐢
let_it_be(:project) { create(:project) }
let_it_be(:maintainer) { create(:user) }
let(:policy) { ProjectPolicy.new(maintainer, project) }

it 'allows admin_project ability' do
  project.add_maintainer(maintainer)

  expect(policy).to be_allowed(:admin_project)
end

# After - fast ⚡
let(:project) { build_stubbed(:project) }
let(:maintainer) { build_stubbed(:user) }
let(:policy) { ProjectPolicy.new(maintainer, project) }

it 'allows admin_project ability' do
  stub_member_access_level(project, maintainer: maintainer)

  expect(policy).to be_allowed(:admin_project)
end

Closes #389247 (closed).

Speed difference? ⚡

The main goal of add this stub is to improve spec speed by avoid creating database records.

Running:

bin/rspec spec/presenters/issue_email_participant_presenter_spec.rb spec/presenters/project_clusterable_presenter_spec.rb

Before

Finished in 14.72 seconds (files took 9.32 seconds to load)
16 examples, 0 failures

Randomized with seed 11517

[TEST PROF INFO] Time spent in factories: 00:06.349 (39.38% of total time)
[TEST PROF INFO] Factories usage

 Total: 68
 Total top-level: 25
 Total time: 00:06.349 (out of 00:22.365)
 Total uniq factories: 10

   total   top-level     total time      time per call      top-level time               name

      19           6        1.8872s            0.0993s             0.8689s               user
      12          12        3.6497s            0.3041s             3.6497s            project
      12           0        1.3762s            0.1147s             0.0000s          namespace
      12           4        2.2075s            0.1840s             1.4823s            cluster
       4           0        0.4266s            0.1067s             0.0000s cluster_provider_gcp
       4           0        0.4826s            0.1206s             0.0000s cluster_platform_kubernetes
       2           2        0.0149s            0.0074s             0.0149s            license
       1           1        0.3333s            0.3333s             0.3333s              group
       1           0        0.0175s            0.0175s             0.0000s namespace_settings
       1           0        0.0174s            0.0174s             0.0000s namespace_ci_cd_settings

After

Finished in 7.62 seconds (files took 20.45 seconds to load)
16 examples, 0 failures

Randomized with seed 26708

[TEST PROF INFO] Time spent in factories: 00:00.032 (0.16% of total time)
[TEST PROF INFO] Factories usage

 Total: 2
 Total top-level: 2
 Total time: 00:00.032 (out of 00:26.185)
 Total uniq factories: 1

   total   top-level     total time      time per call      top-level time               name

       2           2        0.0326s            0.0163s             0.0326s            license

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.