gh-114911: Add CPUStopwatch test helper

[encukou]

A few of our tests measure the time of CPU-bound operation, mainly to avoid quadratic or worse behaviour.
Add a helper to ignore GC and time spent in other processes.

• Issue: test_int.test_denial_of_service failed: took 15ms #114911

[encukou]

@vstinner, do you like this test helper?

[vstinner]

@vstinner, do you like this test helper?

I'm not sure that it's going to fix the root issue: #114911 (comment)

[encukou]

It's not; I want to add it to help diagnosing similar issues (by filtering out unrelated causes).

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit bed30c3 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[encukou]

The WASI buildbot fails because... on WASI time runs a million times faster: #115714

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 0d29c04 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[vstinner]

I'm not sure that it's going to fix the root issue: #114911 (comment)

Please, ignore my comment. I didn't find time to fully review this change and dig into this issue ;-)

[encukou]

The WASI clock has been sacked; tests are green.

@vstinner Thank you for digging in! Any objections to merging this?
(I'm not asking for a full formal review, unless you want to do one -- but I don't want to merge if you're still looking at this.)