PR adds `test-linux-stable-int` and `quick-benchmarks` as github action
jobs. It's a copy of `test-linux-stable-int` and `quick-benchmarks` from
gitlab ci and now it's needed to make a stress test for self-hosted
github runners. `test-linux-stable-int` and `quick-benchmarks` in gitlab
are still `Required` whereas this workflow is allowed to fail.
cc https://github.com/paritytech/infrastructure/issues/46
Alexander Samusev