Add DiversiTest

[ob-ivan]

Adding a development version of DiversiTest: a new utility to run tests with a range of dependencies' versions.

Run composer install, then:

vendor/bin/diversitest

This will clone current working directory to a temporary folder, install dependencies' versions as configured in diversitest.yaml and run phpunit.

You can check the source here: https://github.com/ob-ivan/diversitest

TODO:

• Downgrade symfony/console to 3.*
• Downgrade symfony/process to 3.*

[lezhnev74]

@ob-ivan this is awesome! Let me play with it for a couple of days!