Fix flaky test_steal_twice and test_steal_when_more_tasks

[crusaderky]

• Closes test_steal_twice flaky #4957
• test_steal_twice was almost always failing on high-CPU-count desktops due to unbound number of threads-per-worker
• Change the tests to be meaningful when scheduler-side queueing is on
• Tweak slow marker for the module

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    40 files  ±0      40 suites  ±0   14h 39m 31s ⏱️ + 13m 43s
 4 152 tests ±0   3 972 ✅ +1    178 💤 ±0  2 ❌  - 1 
80 800 runs  +1  76 560 ✅ +1  4 238 💤 +2  2 ❌  - 2 

For more details on these failures, see this check.

Results for commit 597f6b2. ± Comparison against base commit daff1c6.

This pull request removes 1 and adds 1 tests. Note that renamed tests count towards both.

distributed.tests.test_steal ‑ test_steal_when_more_tasks

distributed.tests.test_steal ‑ test_steal_large_dependency

♻️ This comment has been updated with latest results.