ci: add windows testing to CI by lifting OS into its own test matrix …

[filmaj]

• Run tests on Windows in CI.
• Tweak a few failing tests on Windows as a result
  • the instrumentation of user agent in web-api needs changing, as process.title can return a wide variety of results, including it can be completely overridden!
  • some cli-test functionality is skipped on Windows (graceful shutting down of processes / lack of interrupt signal support on Windows), so the relevant test executing this branch of code also needs to be skipped on windows.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.83%. Comparing base (1be7a9e) to head (a92d53b). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2081 +/- ## ========================================== - Coverage 91.83% 91.83% -0.01% ========================================== Files 38 38 Lines 10264 10263 -1 Branches 646 646 ========================================== - Hits 9426 9425 -1 Misses 826 826 Partials 12 12 ``` | [Flag](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2081/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | Coverage Δ | | |---|---|---| | [cli-test](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2081/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.69% <ø> (ø)` | | | [oauth](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2081/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `77.39% <ø> (ø)` | | | [socket-mode](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2081/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `58.22% <ø> (ø)` | | | [web-api](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2081/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.88% <100.00%> (-0.01%)` | :arrow_down: | | [webhook](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2081/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.65% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi#carryforward-flags-in-the-pull-request-comment) to find out more.