Create guest users only when necessary rather than for every spec

These are two examples of a top-level `before` block doing too much.
Only specific specs cared about these guest users, but we were creating
them and their `ProjectMember` records for every single spec that ran.

spec/features/explore/new_menu_spec.rb

 require 'spec_helper'
 
 feature 'Top Plus Menu', feature: true, js: true do
-  let(:user) { create :user }
-  let(:guest_user) { create :user}
+  let(:user) { create(:user) }
   let(:group) { create(:group) }
   let(:project) { create(:project, :repository, creator: user, namespace: user.namespace) }
   let(:public_project) { create(:project, :public) }
 
   before do
     group.add_owner(user)
-    group.add_guest(guest_user)
-
-    project.add_guest(guest_user)
   end
 
   context 'used by full user' do
@@ -102,7 +98,12 @@ feature 'Top Plus Menu', feature: true, js: true do
   end
 
   context 'used by guest user' do
+    let(:guest_user) { create(:user) }
+
     before do
+      group.add_guest(guest_user)
+      project.add_guest(guest_user)
+
       sign_in(guest_user)
     end
 

spec/support/features/issuable_slash_commands_shared_examples.rb

@@ -5,8 +5,6 @@ shared_examples 'issuable record that supports quick actions in its description
   include QuickActionsHelpers
 
   let(:master) { create(:user) }
-  let(:assignee) { create(:user, username: 'bob') }
-  let(:guest) { create(:user) }
   let(:project) { create(:project, :public) }
   let!(:milestone) { create(:milestone, project: project, title: 'ASAP') }
   let!(:label_bug) { create(:label, project: project, title: 'bug') }
@@ -15,8 +13,6 @@ shared_examples 'issuable record that supports quick actions in its description
 
   before do
     project.team << [master, :master]
-    project.team << [assignee, :developer]
-    project.team << [guest, :guest]
 
     sign_in(master)
   end
@@ -57,6 +53,7 @@ shared_examples 'issuable record that supports quick actions in its description
 
     context 'with a note containing commands' do
       it 'creates a note without the commands and interpret the commands accordingly' do
+        assignee = create(:user, username: 'bob')
         write_note("Awesome!\n/assign @bob\n/label ~bug\n/milestone %\"ASAP\"")
 
         expect(page).to have_content 'Awesome!'
@@ -77,6 +74,7 @@ shared_examples 'issuable record that supports quick actions in its description
 
     context 'with a note containing only commands' do
       it 'does not create a note but interpret the commands accordingly' do
+        assignee = create(:user, username: 'bob')
         write_note("/assign @bob\n/label ~bug\n/milestone %\"ASAP\"")
 
         expect(page).not_to have_content '/assign @bob'
@@ -111,8 +109,12 @@ shared_examples 'issuable record that supports quick actions in its description
 
       context "when current user cannot close #{issuable_type}" do
         before do
+          guest = create(:user)
+          project.add_guest(guest)
+
           sign_out(:user)
           sign_in(guest)
+
           visit public_send("namespace_project_#{issuable_type}_path", project.namespace, project, issuable)
         end
 
@@ -146,8 +148,12 @@ shared_examples 'issuable record that supports quick actions in its description
 
       context "when current user cannot reopen #{issuable_type}" do
         before do
+          guest = create(:user)
+          project.add_guest(guest)
+
           sign_out(:user)
           sign_in(guest)
+
           visit public_send("namespace_project_#{issuable_type}_path", project.namespace, project, issuable)
         end
 
@@ -176,6 +182,9 @@ shared_examples 'issuable record that supports quick actions in its description
 
       context "when current user cannot change title of #{issuable_type}" do
         before do
+          guest = create(:user)
+          project.add_guest(guest)
+
           sign_out(:user)
           sign_in(guest)
           visit public_send("namespace_project_#{issuable_type}_path", project.namespace, project, issuable)
@@ -267,6 +276,8 @@ shared_examples 'issuable record that supports quick actions in its description
 
   describe "preview of note on #{issuable_type}" do
     it 'removes quick actions from note and explains them' do
+      create(:user, username: 'bob')
+
       visit public_send("namespace_project_#{issuable_type}_path", project.namespace, project, issuable)
 
       page.within('.js-main-target-form') do