Improve ClientTimeoutSuite

[RafalSumislawski]

This PR solves the following problems:

• The use of QueueTestHead with onFinalizeWeak which is by it's nature racy and leads to flaky tests. (this half solves Redesign tests based on QueueTestHead and onFinalizeWeak blaze#703)
• Incorrect construction of connections, leading to to the IdleTimoutStage not being included in the pipelinie (it used to be added in mkConnection but then removed in MockClientBuilder)
• All tests of idle-timeout passing despite the issue mentioned above. (This is solved by redefining the "Not timeout on only marginally slow request body" in such a way that it would fail if the IdleTimeoutStage wasn't included in the pipeline)

Additionally it:

• improves test coverage by dividing "Idle timeout on slow POST body" into two test cases, one of which is currently failing due to a flaw in blaze client implementation (I think Resourcify blaze client #5385 will solve it).
• reduces amount of code

[danicheg]

Thank you for adding comments for those test suites. So much appreciated.