Add tests for Geo health check

spec/lib/gitlab/geo/health_check_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Geo::HealthCheck do
+  let!(:secondary) { create(:geo_node, :current) }
+
+  subject { described_class }
+
+  describe '.perform_checks' do
+    it 'returns an empty string when not running on a secondary node' do
+      allow(Gitlab::Geo).to receive(:secondary?) { false }
+
+      expect(subject.perform_checks).to be_blank
+    end
+
+    it 'returns an error when configuration file is missing for tracking DB' do
+      allow(Rails.configuration).to receive(:respond_to?).with(:geo_database) { false }
+
+      expect(subject.perform_checks).not_to be_blank
+    end
+
+    it 'returns an error when Geo database version does not match the latest migration version' do
+      allow(subject).to receive(:get_database_version) { 1 }
+
+      expect(subject.perform_checks).not_to be_blank
+    end
+
+    it 'returns an error when latest migration version does not match the Geo database version' do
+      allow(subject).to receive(:get_migration_version) { 1 }
+
+      expect(subject.perform_checks).not_to be_blank
+    end
+  end
+end