Test lock updates

[samschott]

• Fix an issue with the test lock being deleted in cleanup before test case.
• Simplify pytest fixtures.
• Aquire lock on per-session instead of per-test-case.
• Aquire lock for a max timeout of 10 min and renew for another 10 min for each test case.

[codecov[bot]]

Codecov Report

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

Project coverage is 79.03%. Comparing base (7666b12) to head (1e58da3). Report is 45 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1051 +/- ## =========================================== + Coverage 55.86% 79.03% +23.17% =========================================== Files 42 42 Lines 7551 7551 =========================================== + Hits 4218 5968 +1750 + Misses 3333 1583 -1750 ``` | [Flag](https://app.codecov.io/gh/samschott/maestral/pull/1051/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SamSchott) | Coverage Δ | | |---|---|---| | [pytest](https://app.codecov.io/gh/samschott/maestral/pull/1051/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SamSchott) | `79.03% <ø> (+23.17%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SamSchott#carryforward-flags-in-the-pull-request-comment) to find out more.

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