Add automated travis tests and cherry pick #10

[convenient]

This is quite a big PR, but I believe it all makes sense.

The reason for this PR are twofold

• To make testing feedback like Use plugin after place instead of observer after save #10 easier and automated as far as possible.
• This is necessitated as magento have closed Anyway to disable stock reservation? magento/inventory#2269 meaning this is now going to stay in the core and we need to make sure this module works on all subsequent versions. We don't want to have to manually re-test this once per quarter if we can avoid it at all.

Travis will run 2 builds

• one against 2.3.3 which we know the issue was present in
• one against the latest version of magento

Addition of test suite

I have added a codeception test suite which creates a product, adds to basket, checks out and then does assertions for each of the features listed in the README.md

See

• dev/codeception.MD
• dev/tests/acceptance/CheckoutCest.php

In order to do this I had to move all the existing code into the src directory and update the composer.json. This is because otherwise magento was detecting PHP code in the dev directory and trying to include it in the di:compile

The suite spins up a fully fledged instance of Magento 2 by using https://github.com/AmpersandHQ/travis-vanilla-magento which is ultimately powered by https://store.fooman.co.nz/blog/no-authentication-needed-magento-2-mirror.html

Cherry picking #10 - Use plugin after place instead of observer after save

I added a test to capture the error reported in #10 see 454faf8

You can see it failed here https://travis-ci.org/github/AmpersandHQ/magento2-disable-stock-reservation/builds/665961249

1) CheckoutCest: Repeat saves of order do not decrement quantity un
 Test  tests/acceptance/CheckoutCest.php:repeatSavesOfOrderDoNotDecrementQuantityUn
 Step  Assert equals 98,"93.0000","The quantity should have gone down by 1"
 Fail  The quantity should have gone down by 1
Failed asserting that '93.0000' matches expected 98.0.

When I cherry picked in the commit from #10 the suite went green. See 3c48dbd and https://travis-ci.org/github/AmpersandHQ/magento2-disable-stock-reservation/builds/665975512

[convenient]

The PR build has failed because of merging shenanigans. It runs as the target branch (master) so pulls in the dev-master branch which doesnt have the fix.

The push build uses the current branch which has the fix.

I've disabled the PR builds in travis config so that future bug fixes don't look so weird.

Once this gets merged I'm confident master will be green.