Only run these tests on one CI builder (not all platforms) though. This
is extremely resource intensive since it rebuilds libstd. Currently this
does not share a build directly like before because the number of tests
are supposed to be small, but if necessary we can add that in later too.
I've noticed recently that the incremental compile time for our test
suite has felt like it's increased quite a bit. I think one reason is
that everything has to go through `#[cargo_test_macro]` unconditionally
on all incremental builds, and wow do we have a lot of tests being
pumped through that macro.
Instrumenting the macro a little bit shows that we spend nearly 2.5
seconds on each compilation simply executing this macro (note that it's
in debug mode as well, not release since we typically don't execute
tests in release mode.
This commit instead drops the usage of `syn` and `quote` in favor of a
"raw procedural macro" which is much more optimized for just our use
case, even in debug mode. This drops the collective time spent in the
macro to 0.2 seconds, even in debug mode!
These tests take a good amount of time to run locally and they're also
causing a lot of dependencies to get pulled into rust-lang/rust, so
let's have a separate crate that we just test on our own CI
