Add matcher to match elements by ids

Initially added in gitlab-org/gitlab-ee!4864 and
gitlab-org/gitlab-ee!4689.

spec/support/matchers/match_ids.rb

+RSpec::Matchers.define :match_ids do |*expected|
+  match do |actual|
+    actual_ids = map_ids(actual)
+    expected_ids = map_ids(expected)
+
+    expect(actual_ids).to match_array(expected_ids)
+  end
+
+  description do
+    'matches elements by ids'
+  end
+
+  def map_ids(elements)
+    elements = elements.flatten if elements.respond_to?(:flatten)
+
+    if elements.respond_to?(:map)
+      elements.map(&:id)
+    elsif elements.respond_to?(:id)
+      [elements.id]
+    else
+      raise ArgumentError, "could not map elements to ids: #{elements}"
+    end
+  end
+end