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

@jasonroelofs & @fw42 for review

## Problem

The RenderProfilingTest #test_profiling_times_the_entire_render test was failing inconsistently on jruby due to Time.now having millisecond rather than microsecond precision.  As a result, the total_render_time can end up being 0 since render time can complete in less than a microsecond.

## Solution

Allow total_render_time to be 0 in the test.
It doesn't get initialized, it is calculated using `@render_end_at - @render_start_at`, where both of those variables are set using `Time.now`.  This can still help catch subtracting in the wrong order and getting a negative value.