chore(test): setup a "flappy tests" system

[moul]

• Setup system in Makefile
• Setup new testutils helpers
• Update CI

to get a list of current flappy tests, you can run:

$ git grep "func TestFlappy"
pkgs/bft/blockchain/reactor_test.go:func TestFlappyBadBlockStopsPeer(t *testing.T) {
pkgs/bft/consensus/replay_test.go:func TestFlappyHandshakeReplayNone(t *testing.T) {
pkgs/bft/mempool/reactor_test.go:func TestFlappyBroadcastTxForPeerStopsWhenReactorStops(t *testing.T) {
pkgs/p2p/transport_test.go:func TestFlappyTransportMultiplexAcceptNonBlocking(t *testing.T) {