[PATCH v2] selftests: Add version file to kselftest

Thanks for your patience for review! I've updated the patch to make it easier to understand. Specifically, `stdout` of `git describe` is stored into `VERSION` variable, then write into `VERSION` file rather than print on-screen. Please refer below for the sample output and the content in `VERSION`.

``` Emit Tests for drivers/net Skipping non-existent dir: drivers/net/hw Version saved to /tmp/ksft-net-drv/VERSION make: Leaving directory '/home/tianyicui/test/nipa/net-next/tools/testing/selftests' ➜ net-next git:(add_kversion) ✗ cat /tmp/ksft-net-drv/VERSION v6.15-12423-g7ebe76e39b4c ```