Clear memoized snowplow class

This spec mutates @snowplow to store SnowplowMicro which is only
available in development. This clears the memoization after the spec
runs.

spec/lib/gitlab/tracking_spec.rb

@@ -13,6 +13,10 @@ RSpec.describe Gitlab::Tracking do
     described_class.instance_variable_set("@snowplow", nil)
   end
 
+  after do
+    described_class.instance_variable_set("@snowplow", nil)
+  end
+
   describe '.options' do
     shared_examples 'delegates to destination' do |klass|
       before do