Unintended speed dependence in should_have_diverse_proposable_blocks_with_stocked_buffer test

[Sainath Sherigar]

The should_have_diverse_proposable_blocks_with_stocked_buffer depends on timestamps collected at runtime and some guesses for execution time. It has exhibited some flakiness in the past that has been addressed, but in its current iteration it fails in debug mode, while it passes with --release flags.