test: adopt `testing/synctest` once go 1.26 is released

See this blog post for more info: https://go.dev/blog/synctest

This should significantly speed up our test suite, which currently does a lot of relatively complex time management and manual sleeping for a variety of our timing-based endpoints.

Since it was only made generally available in 1.25, we can't adopt it till 1.26 is released, to maintain our support for the two most recent releases.