feat: add tests for node/heart.ts (#5122)

* refactor(heart): extract logic into heartbeatTimer fn

To make it easier to test, I extract heartbeatTimer into it's own
function.

* feat(testing): add tests for heart.ts

* fixup

* fixup!: remove unneeded heart call

* Update src/node/heart.ts

Co-authored-by: Asher <ash@coder.com>

* fixup!: use mockResolvedValue everywhere

* fixup!: add stat test for timestamp check

Co-authored-by: Asher <ash@coder.com>