How to test in a collaborative environment?

My question is how to develop and test a Web application in a small team (2-3 people)? Imagine such a scenario:

A small team of 2-3 people are working together on a Web application. The Web server is on one computer, maybe at someone's home or a hosting environment, i.e. Google App Engine or Redhat openshift, and the source code is hosted in github or bitbucket. Developers check in and out code from github/bigbucket. Since the Web application is actually hosted in Google App Engine or Openshift, then a few questions arise:

1) How to move code between github/bigbucket and Google App Engine/Openshift? How often is the code synced between the two?
2) How to test the code, to make sure it will be running fine in the whole application? Because we can't test it on developer's local computer or on github/bitbucekt?
3) What't the best practice of setting up a collaborative development environment?

Thanks for your comments, and insights!