Disabled tests in node_test.go need to be updated

## Summary

The following tests have been disabled some time ago, because they are "Randomly failing".
Things got worse when in https://github.com/algorand/go-algorand/pull/893 ExtendPeerList was removed, which was used by these tests.

node/node_test.go-func TestSyncingFullNode(t *testing.T) {
node/node_test.go: t.Skip("This is failing randomly again - PLEASE FIX!")

node/node_test.go-func TestInitialSync(t *testing.T) {
node/node_test.go: t.Skip("flaky TestInitialSync ")

node/node_test.go-func TestSimpleUpgrade(t *testing.T) {
node/node_test.go: t.Skip("Randomly failing: node_test.go:~330 : no block notification for account. Re-enable after agreement bug-fix pass")

## Scope/Requirements

Update the test to establish the network connections without using ExtendPeerList.
Determine why the tests are randomly failing and fix it.
Enable the tests.

## Urgency/Relative Priority

This is not urgent, since these test were disabled some time ago.
It is important to fix and enable them, since the delay causes the test to further drift away from the source code.