[PATCH v2] selftests: Add version file to kselftest

[Tianyi Cui]

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
```