Restrict Geo tests to PostgreSQL

spec/controllers/admin/geo_nodes_controller_spec.rb

 require 'spec_helper'
 
-describe Admin::GeoNodesController do
+describe Admin::GeoNodesController, :postgresql do
   shared_examples 'unlicensed geo action' do
     it 'redirects to the license page' do
       expect(response).to redirect_to(admin_license_path)

spec/lib/gitlab/geo/health_check_spec.rb

 require 'spec_helper'
 
-describe Gitlab::Geo::HealthCheck do
+describe Gitlab::Geo::HealthCheck, :postgresql do
   let!(:secondary) { create(:geo_node, :current) }
 
   subject { described_class }
 
   describe '.perform_checks' do
-    before do
-      skip("Not using PostgreSQL") unless Gitlab::Database.postgresql?
-    end
-
     it 'returns a string if database is not fully migrated' do
       allow(Gitlab::Geo).to receive(:secondary?) { true }
       allow(Gitlab::Geo).to receive(:secondary_role_enabled?).and_return(true)

spec/serializers/geo_node_status_entity_spec.rb

 require 'spec_helper'
 
-describe GeoNodeStatusEntity do
+describe GeoNodeStatusEntity, :postgresql do
   let(:geo_node_status) do
     GeoNodeStatus.new(
       id: 1,