test: Skip flaky tests

[vdusek]

• Test test_keep_alive is flaky #1649
• Test test_adaptive_context_query_selector_parsel is flaky #1650
• Test test_isolation_cookies is flaky #1651
• Test test_timeout_in_handler is flaky #1652
• Test test_autoscales is flaky #1655
• Test test_new_page_with_each_plugin is flaky #1660

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.50%. Comparing base (bff7260) to head (55f92b1).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1661   +/-   ##
=======================================
  Coverage   92.50%   92.50%           
=======================================
  Files         157      157           
  Lines       10440    10440           
=======================================
  Hits         9657     9657           
  Misses        783      783           

Flag	Coverage Δ
unit	92.50% <ø> (ø)

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