This commit adds a simple test job for illumos using Buildomat. I'd like
to make some more improvements and add docs before merging this, but I
want to see if it even works first.

The illumos version of `echo(1)` doesn't support `-n`, so it interprets
it literally as part of the string to echo, making the test fail. I've
changed it to use `printf(1)` with no flags args to get the same
behavior.

I also changed the name of the test, since it doesn't actually spawn
`echo(1)` anymore :)

## Motivation

In `tokio::task::local`, there's a
`LocalState::assert_called_from_owner_thread` function, which checks
that the caller's thread ID matches that of the thread on which the
`LocalSet` was created. This assertion is disabled on FreeBSD and
OpenBSD, with a comment indicating that this is because those systems
have "some weirdness around thread-local destruction". If memory serves,
I believe the "weirdness" was related to the order in which
thread-locals are destroyed relative to each other, or something along
those lines.

Upon running the tests on illumos, there appears to be [a similar
panic][1] in the `task_local_set_in_thread_local` due to this assertion
assertion while dropping a `LocalSet` which lives in a thread-local.
This leads me to believe that illumos, and presumably Solaris, also
exhibit the same "weirdness". This wouldn't be too surprising, given the
shared heritage of those systems.

## Solution

This commit adds `target_os="illumos"` and `target_os="solaris"` to the
`cfg` attribute that disables the assertion on systems determined to
have weirdness. This fixes the test panicking on illumos.

In the future, it might be worthwhile to look into changign the
assertion to only be disabled on weirdness-having systems _while
dropping the `LocalSet`_, rather than all the time. That way, we can
still check other accesses. But, I wanted to make the minimum change
necessary to fix it here before messing around with that.

[1]: https://buildomat.eng.oxide.computer/wg/0/details/01J592RB0JR203RGGN0RYHJHMY/CHieo1Uee7qzRVyp811YBl0MvXGO3i0QA9ezPaFWj6hf6P3P/01J592RSWCNX1MCFYGW74AHVH6#S1954