Fix rubocop warnings in spec/lib and spec/tasks

spec/lib/gitlab/backend/grack_auth_spec.rb

@@ -151,14 +151,14 @@ describe Grack::Auth do
                 end
                 it "repeated attempts followed by successful attempt" do
-                  for n in 0..maxretry do
+                  maxretry.times.each do
                     expect(attempt_login(false)).to eq(401)
                   end
                   expect(attempt_login(true)).to eq(200)
                   expect(Rack::Attack::Allow2Ban.banned?(ip)).to be_falsey
-                  for n in 0..maxretry do
+                  maxretry.times.each do
                     expect(attempt_login(false)).to eq(401)
                   end
                 end

spec/lib/gitlab/o_auth/user_spec.rb

@@ -19,10 +19,6 @@ describe Gitlab::OAuth::User do
     let!(:existing_user) { create(:omniauth_user, extern_uid: 'my-uid', provider: 'my-provider') }
     it "finds an existing user based on uid and provider (facebook)" do
-      # FIXME (rspeicher): It's unlikely that this test is actually doing anything
-      # `auth` is never used and removing it entirely doesn't break the test, so
-      # what's it doing?
-      auth = double(info: double(name: 'John'), uid: 'my-uid', provider: 'my-provider')
       expect( oauth_user.persisted? ).to be_truthy
     end

spec/tasks/gitlab/backup_rake_spec.rb

@@ -159,7 +159,7 @@ describe 'gitlab:app namespace rake task' do
     end
     it "does not contain skipped item" do
-      tar_contents, exit_status = Gitlab::Popen.popen(
+      tar_contents, _exit_status = Gitlab::Popen.popen(
         %W{tar -tvf #{@backup_tar} db uploads repositories builds}
       )