Mitigate flakyness in test_slurm_driver

[berland]

The flakyness was easily reproducible with --count=100 -n 16, but after adding reruns the flakyness cannot be reproduced, so assuming reruns is sufficient (this was with mocked Slurm system, not with the --slurm option to pytest).

Since we have added reruns, we can reduce the job_kill_window when doing real integration tests also.

Issue Resolves #8260

Approach pytest.mark.flaky(reruns=3)

• [ ] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [x] Commit history is consistent and clean, in line with the contribution guidelines.
• [x] Make sure tests pass locally (after every commit!)

When applicable

• [ ] When there are user facing changes: Updated documentation
• [ ] New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• [ ] Large PR: Prepare changes in small commits for more convenient review
• [ ] Bug fix: Add regression test for the bug
• [ ] Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.04%. Comparing base (b3b5950) to head (91ad6e2). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8262 +/- ## ========================================== + Coverage 87.02% 87.04% +0.02% ========================================== Files 378 378 Lines 23947 23952 +5 Branches 624 618 -6 ========================================== + Hits 20840 20850 +10 + Misses 3029 3021 -8 - Partials 78 81 +3 ```

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