rust: kunit: support KUnit-mapped `assert!` macros in `#[test]`s

The KUnit `#[test]` support that landed recently is very basic and does
not map the `assert*!` macros into KUnit like the doctests do, so they
panic at the moment.

Thus implement the custom mapping in a similar way to doctests, reusing
the infrastructure there.

In Rust 1.88.0, the `file()` method in `Span` may be stable [1]. However,
it was changed recently (from `SourceFile`), so we need to do something
different in previous versions. Thus create a helper for it and use it
to get the path.

With this, a failing test suite like:

#[kunit_tests(my_test_suite)]
mod tests {
use super::*;

#[test]
fn my_first_test() {
assert_eq!(42, 43);
}

#[test]
fn my_second_test() {
assert!(42 >= 43);
}
}

will properly map back to KUnit, printing something like:

[ 1.924325] KTAP version 1
[ 1.924421] # Subtest: my_test_suite
[ 1.924506] # speed: normal
[ 1.924525] 1..2
[ 1.926385] # my_first_test: ASSERTION FAILED at rust/kernel/lib.rs:251
[ 1.926385] Expected 42 == 43 to be true, but is false
[ 1.928026] # my_first_test.speed: normal
[ 1.928075] not ok 1 my_first_test
[ 1.928723] # my_second_test: ASSERTION FAILED at rust/kernel/lib.rs:256
[ 1.928723] Expected 42 >= 43 to be true, but is false
[ 1.929834] # my_second_test.speed: normal
[ 1.929868] not ok 2 my_second_test
[ 1.930032] # my_test_suite: pass:0 fail:2 skip:0 total:2
[ 1.930153] # Totals: pass:0 fail:2 skip:0 total

Link: https://github.com/rust-lang/rust/pull/140514 [1]
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250502215133.1923676-2-ojeda@kernel.org
[ Required `KUNIT=y` like for doctests. Used the `cfg_attr` from the
TODO comment and clarified its comment now that the stabilization is
in beta and thus quite likely stable in Rust 1.88.0. Simplified the
`new_body` code by introducing a new variable. Added
`#[allow(clippy::incompatible_msrv)]`. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>