fix goroutine leaks in core tests

[darioush]

Why this should be merged

Tests should not leak goroutines, this slows down the environment for other tests.

How this works

calls Stop methods or uses test stubs which do not launch goroutines instead of implementation.

How this was tested

UTs + goleak

How is this documented

N/A

[darioush]

waiting on this till https://github.com/ava-labs/coreth/pull/273/files is merged