tools/regression: Expand stdatomic conformance tests

Extend the existing test utility to cover recent stdatomic
changes and improve validation of interface semantics,
compile-time assertions, and behavioural conformance.

Signed-off-by: Faraz Vahedi <kfv@kfv.io>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/2185