Fix flaky test which assumes total_render_time can't be 0.

jruby has millisecond precision for Time.now, so total_render_time can be 0
due to this lack of precision.
Merge pull request #449 from Shopify/fix-flaky-total-render-time-test

Fix flaky test which assumes total_render_time can't be 0.
Remove Liquid::FilterNotFoundError since it is never raised.
Merge pull request #448 from Shopify/remove-unused-filter-not-found-error

Remove Liquid::FilterNotFoundError since it is never raised.