chore: use pubsub interface

[vasco-santos]

This PR moves away from libp2p-pubsub base implementation and uses the new (still in PR) interface-pubsub.

The interface-pubsub provides some advantages over libp2p-pubsub:

• common test suite used by pubsub routers, decreasing tests duplication and future needs in new routers
• guarantees that the pubsub routers have the expected API and behaviour (no need to have a pubsub abstraction in js-libp2p per PubSub abstraction API libp2p/js-libp2p-pubsub#68)
• more info RFC: pubsub-interface libp2p/js-libp2p-interfaces#53

The current base pubsub implementation in the interface is mostly the content of https://github.com/libp2p/js-libp2p-pubsub/tree/v0.6.x with some changes in the constructor.

This PR aims to:

• Use interface-pubsub with its base implementation and test suite
• Remove unnecessary tests
• Heartbeat start/stop errors removed
• Handler for subscribe/unsubscribe feature added (remove libp2p abstraction that mostly adds this)

Note: The interface still needs to be updated with the content of pubsub.js in this repo.

Needs:

• feat: interface pubsub libp2p/js-libp2p-interfaces#60
• chore: update pubsub libp2p/js-libp2p-floodsub#108

[vasco-santos]

I was initially planning to remove all this file. These tests are mostly redundant now, but all the mesh related expects are an extension from the interface tests, as they are specific to gossipsub.
We can create new tests to guarantee that the mesh content is properly tested, or keep these tests for now and open an issue to tackle this test refactor later as it is not a priority

[codecov-commenter]

Codecov Report

Merging #122 into gsv1.1 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           gsv1.1     #122   +/-   ##
=======================================
  Coverage   91.66%   91.66%           
=======================================
  Files           2        2           
  Lines          24       24           
=======================================
  Hits           22       22           
  Misses          2        2           

Impacted Files	Coverage Δ
test/utils/index.js	81.81% <ø> (ø)

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[vasco-santos]

While this still depends on some branches, we can perhaps get this merged as this will not go to master yet.
cc @wemeetagain