`<filesystem>`: Consider setting up the 1ES Hosted Pool with long path support for test coverage

We don't have automated test coverage of `<filesystem>`'s long path support - if I recall correctly, this is because our internal test machines aren't under the STL's control, so we can't assume anything about their OS support. Now that we're on GitHub, I believe we could add long path support to our 1ES Hosted Pool scripts, and then conditionally add test coverage.

Prompted by #1921.