The test suite for this repo has quite a lot of tests, and it is
difficult to tell which contribute the most to the long CI runtime.
libtest does have an unstable flag to report test times, but that is
inconvenient to use because it needs to be passed only to libtest
binaries.
Switch to cargo-nextest [1] which provides time reporting and, overall,
a better test UI. It may also improve test runtime, though this seems
unlikely since we have larger test binaries with many small tests
(nextest benefits the most when there are larger binaries that can be
run in parallel).
For anyone running locally without, `run.sh` should still fall back to
`cargo test` if `cargo-nextest` is not available.
This diff includes some cleanup and consistency changes to other
CI-related files.
[1]: https://nexte.st
CI for aarch64 Linux is significantly slower than the others. Adjust how
iteration selection is done to better handle this case, which also
simplifies things.
Also set the `EMULATED` environment variable in Docker to be more
accurate, and reindents run-docker.sh.
