Tests for stable versions

[vankoven]

For a proper tests we have to use three entities of the correct versions:

• Patched kernel
• Functional tests
• Tempesta itself

Depending on Tempesta version, versions of other entities may differ. E.g. Current master uses 4.14 kernel, while 0.5 version uses 4.9 kernel. When a new PR is created, it should automatically choose the right versions based on target branch in the PR.

[krizhanovsky]

From #56:

Master branch of tests is not compatible with older releases. There are two sources of the problem: new features and changes in the configuration file format. It'd be better to have one test code base for all the versions. New features can be disabled in tests_disabled.json with a special version field, e.g. "version" : "0.7" means that the test is disabled for Tempesta version below 0.7. In the new framework, the full Tempesta configuration is listed in the test description file, not compiled from parts, so we can give very specific configuration for exact Tempesta version. Some good and generic solution is required.

nginx-test tests check the nginx SUT version and just do nothing if the test is for newer feature than the Nginx version.