Arm backend: Align handling of flaky arm unittests (#7669)

* Align handling of flaky arm unittests

- Removes uses of torch_manual_seed which previously fixed the random state.
- Adds pytest plugin pytest-rerunfailures to mark flaky tests.
- Refactors flaky tests to use data generators in favor of pregenerated data, which ensures that data is randomized between reruns.
- Updates layer_norm testcase to use same qtol value for TOSA/EthosU targets.

Note that fixing the randomness may lead to that we will see more flakyness in CI, this will have to be adressed with the flaky mark on a case by case basis over time.

* Fix expectedFailure