ctest: add tests for aliases, structs, unions, as well as a test crate.

[mbyx]

Description

This PR adds support for testing aliases, structs, and unions. It additionally ports ctest-test to use ctest-next.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI