Implement specified timeout for slow doctests

[user202729]

Fixes #39569

Also add some doctest and show the time taken on GitHub annotation, for convenience. (hopefully someone would look at it once the false positive/noise are dealt with…)

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[user202729]

Note that https://doc.sagemath.org/html/en/developer/coding_basics.html#special-markup-to-influence-doctests requires # long time for tests that requires > 1 second to run, so someone would need to run --warn-long 1 and add the tag in a future pull request. Note that if it takes less than around 20 seconds then you should probably not use this feature, since the default time limit of # long time (without anything else in parentheses) is 30 seconds.

[user202729]

A problem is there are hundreds of occurrences of # long time (2s) and such scattered in the source code, but they are often not meant to set the time limit. In particular, this PR can make more warnings to be raised if some sage: test_something() # long time (2s) actually takes 3 seconds.

I think we should introduce a new syntax sage: test_something() # long time (limit 1s, [optional explanation]), so that it does not conflict with the existing ones. Then apply that on those that may takes longer than 30 seconds.

Thoughts?