Separate deployProxy and upgradeProxy tests

Currently the tests for Truffle and buidler have deployProxy and upgradeProxy in the same script. In production they will be used in separate scripts as an upgrade will happen sometime after a deploy.

The existing upgrades library shows a single script doing deploy and upgrade and this has caused confusion as there aren't examples showing these distinct steps.

The tests act as examples (and will likely be used as the basis for documentation examples), so we should show how they intend to be used.

For example in Truffle we could use something like the following:

2_deploy.js

const Greeter = artifacts.require('Greeter');
const GreeterV2 = artifacts.require('GreeterV2');

const { deployProxy } = require('@openzeppelin/upgrades-truffle');

module.exports = async function (deployer) {
  await deployProxy(Greeter, ['Hello Truffle'], { deployer });
};

3_upgrade.js

const Greeter = artifacts.require('Greeter');
const GreeterV2 = artifacts.require('GreeterV2');

const { upgradeProxy } = require('@openzeppelin/upgrades-truffle');

module.exports = async function (deployer) {
  const greeter = await Greeter.deployed();
  await upgradeProxy(greeter.address, GreeterV2, { deployer });
};

Inspect the existing Truffle and buidler test scripts that currently combine deployProxy and upgradeProxy. Separate the deployment and upgrade steps into distinct scripts for each framework, then run the affected tests to confirm both workflows still work and provide separate examples.