Add tests for ExclusiveLease

spec/lib/gitlab/exclusive_lease_spec.rb

+require 'spec_helper'
+
+describe Gitlab::ExclusiveLease do
+  it 'is exclusive' do
+    lease = Gitlab::ExclusiveLease.new(unique_key, timeout: 3600)
+    expect(lease.try_obtain).to eq(true)
+    expect(lease.try_obtain).to eq(false)
+  end
+
+  it 'expires' do
+    timeout = 1
+    lease = Gitlab::ExclusiveLease.new(unique_key, timeout: timeout)
+    lease.try_obtain
+    sleep(2 * timeout)
+    expect(lease.try_obtain).to eq(true)
+  end
+
+  def unique_key
+    SecureRandom.hex(10)
+  end
+end