[Backport 1.x] Fix creating a new comparison date when hashing step s…

…leep IDs (inngest#220)

Fixes a bug where we use two different `Date` objects to represent
"now," breaking `step.sleep()` hashing in _very rare_ circumstances.

Adds some tests that try to give the problem an opportunity to appear;
it's hard to cause the problem to appear due to it being temporally
reliant.

(cherry picked from commit cc3929d)