Tests for the hard fork summary.

This module verifies the property that /no matter how the summary is constructed/, as long as it satisfies its invariants, we should have roundtrip properties:

• Converting time to a slot and then back to time should be an identity (modulo the time spent in that slot).
• Converting a slot to time and then back should be an identity.
• Converting slot to an epoch and then back to a slot should be an identity (modulo the time spent in that epoch).
• Converting an epoch to a slot and then back should be an identity.