Add an overload of `confirmation()` with a timeout

[grynspan]

DO NOT MERGE: This PR is potentially incompatible with parallelized testing because tests that run in parallel compete for computing resources and a test that only takes (e.g.) 5ms of CPU time may take much more wall-clock time to actually complete. This is extra-true in CI or VM environments.

Checklist:

• [x] Code and documentation should follow the style of the Style Guide.
• [x] If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test

[Ky-MLB]

Is this still being worked on?

[grynspan]

This PR is a proof-of-concept. At this time, I'm not planning to merge it. See the PR description for more details.