Changes the handling of the UR_LEVEL_ZERO_INCLUDE_DIR environment variable. The user can provide any direct path to the include directory containing level zero headers. No 'level-zero' subdirectory appended to this path anymore.

Note that the path to the include directory has to end with a slash, like ie. "level-zero/include/".

instead of running fuzztests in CI on the null adapter.

- Properly set up and trigger the test kernel within the fuzz test.
- Change the test kernel used from "bar" to "fill".
- Refactor env vars used in test.