feat(test2): Provide a #[test] macro

[epage]

This is part of #13

I went with ctor because

• Divan ran into issues with linkme, see nvzqz/nvzqz/divan/pull/32divan#32
• It doesn't involve proc-macros or build scripts

Like ctors optional proc-macro, I kept out proc-macro minimal to experiment with what it might take to use declarative attributes when they are ready.

I put TESTS registry of all tests in callers crate via #[libtest2::main] to simulate the lack of cross-crate registration which is the initial proposal for rust-lang/testing-devex-team#3. If we had cross-crate registration, we could instead expose a pub fn main() {} that people could import and not need #[libtest2::main] at all. We could experiment with putting it in a prelude.

Known issues:

• Only () is allowed as a return type
• No #[should_panic]
• No #[ignore]
• Reported order for tests is weird

[coveralls]

Pull Request Test Coverage Report for Build 18571071689

Details

• 17 of 91 (18.68%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 30.432%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/distributed_list/src/list.rs	14	25	56.0%
crates/libtest2/src/case.rs	0	12	0.0%
crates/libtest2-proc-macro/src/lib.rs	0	51	0.0%

Totals
Change from base Build 18564295825:	-0.7%
Covered Lines:	500
Relevant Lines:	1643

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💛 - Coveralls