Add optimization-level option in benchmark tests

[mvasiljevicTT]

In benchmark/tt-xla/conftest.py there is support for --optimization-level fixture, but it is not used in the tests.

Running tests with --optimization-level 0 will seem to work, but actually use default value 1, which is misleading.

[mvasiljevicTT]

Not crucial in any way, just would be nice if it was either added or removed. Can wait for after tt-xla migration.