What versions of PHP are used in CiviCRM's internal QA?
At time of writing, all PRs tests and scheduled tests run under Ubuntu 12.04 (PHP 5.3.10). There is an effort underway to introduce a test node for Ubuntu 16.04 (PHP 7.0) (although details are still a bit in flux).
When you inspect the results of a test run, you can get some details (such as the node name and console output). The node names follow a convention -- test-ubu1204-5 is running Ubuntu 12.04 (precise); test-ubu1604-1 is running Ubuntu 16.04 (xenial). 


This tells you the general platform and hints at the PHP version. I'd like to keep these generally aligned with upstream versions.
As a somewhat stronger statement, I want to keep the test nodes aligned with civi-download-tools --full, which autoinstalls system requirements on some common builds of Ubuntu/Debian. If you know the target platform, you can lookup a more specific list of packages.