Fix flaky tests on historical vulnerability statistic specs

There were two problems with the previous specs;

1) They tended to fail at runs around midnight
2) `Date.today` does not respect to time zone configuration of Rails
which makes it incompatible with the value of `Time.zone.now` which is
used by Rails internally

Mocking time and using `Date.current` which respects the time zone
fixes the above issues.

ee/spec/factories/vulnerabilities/historical_statistics.rb

@@ -4,6 +4,6 @@ FactoryBot.define do
   factory :vulnerability_historical_statistic, class: 'Vulnerabilities::HistoricalStatistic' do
     project
     letter_grade { 'a' }
-    date { Date.today }
+    date { Date.current }
   end
 end

ee/spec/services/vulnerabilities/historical_statistics/adjustment_service_spec.rb

@@ -5,6 +5,10 @@ require 'spec_helper'
 RSpec.describe Vulnerabilities::HistoricalStatistics::AdjustmentService do
   let_it_be(:project) { create(:project) }
 
+  around do |example|
+    travel_to(Date.current) { example.run }
+  end
+
   describe '.execute' do
     let(:project_ids) { [1, 2, 3] }
     let(:mock_service_object) { instance_double(described_class, execute: true) }
@@ -37,7 +41,7 @@ RSpec.describe Vulnerabilities::HistoricalStatistics::AdjustmentService do
         'info' => 0,
         'unknown' => 0,
         'letter_grade' => 'f',
-        'date' => Date.today.to_s
+        'date' => Date.current.to_s
       }
     end
 

ee/spec/services/vulnerabilities/historical_statistics/update_service_spec.rb

@@ -30,6 +30,10 @@ RSpec.describe Vulnerabilities::HistoricalStatistics::UpdateService do
       end
 
       context 'when there is already a record in the database' do
+        around do |example|
+          travel_to(Date.current) { example.run }
+        end
+
         it 'changes the existing historical statistic entity' do
           historical_statistic = create(:vulnerability_historical_statistic, project: project, letter_grade: 'c')