test: fix flaky Windows CI failures in statistics and browser-pool tests

[vdusek]

Two unit tests flaked on a master push build (run, windows-latest, Python 3.10).

• test_crawler_intermediate_statistics — poll_until_condition(lambda: crawler.statistics.active) used the helper's default 5s ceiling. Under xdist load on a slow Windows runner, crawler startup didn't reach the active state in time, so the poll returned False. Pass a generous timeout=30 (the happy path still returns in <1s, so there's no cost on green runs).
• test_with_default_plugin_constructor — launching a real Firefox browser timed out even with operation_timeout=60s while competing with other xdist workers on the constrained Windows runner. The existing @run_alone_on_mac only isolated macOS, so Windows still ran it in the parallel pool. Upgraded to @pytest.mark.run_alone so the heavy browser launch gets the full machine on every platform; the 60s timeout is kept.

Both flakes are Windows-resource-specific, so local Linux re-runs (10/10 and 5/5 green) confirm the fixes are valid but can't reproduce the original failure mode — CI re-verifies the Windows path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.02%. Comparing base (76927d7) to head (583b47a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1947      +/-   ##
==========================================
+ Coverage   92.98%   93.02%   +0.04%     
==========================================
  Files         167      167              
  Lines       11712    11712              
==========================================
+ Hits        10890    10895       +5     
+ Misses        822      817       -5     

Flag	Coverage Δ
unit	93.02% <ø> (+0.04%)	⬆️

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