Split tests into individual jobs.

[sobomax]

This is part I of two part PR (the other part is in the OpenSIPS/SIPssert repo). It basically splits one monolithic job into individual scenarios to run in parallel. This has the following benefits:

• Test complete much faster. From 30 minutes down to 6-7 minutes;
• Logs are split on per-scenario basis making debugging much easier;
• If one of many scenarios fail, it's very obviously where it fails;
• Different OS/OpenSIPS versions can be tested in parallel if needed.

The matrix is controlled by the matrix.yml file in the root of the tests repo.

[sobomax]

P.S. The action would fail until both parts are merged.