* Restore POSIX signal handling on MacOS behind a feature flag
As described in Issue #3052, the switch to Mach Exception handling
removed `unix::StoreExt` from the public API of crate on MacOS.
That is a breaking change and makes it difficult for some
application to upgrade to the current stable Wasmtime.
As a workaround this PR introduces a feature flag called
`posix-signals-on-macos` that restores the old behaviour on MacOS.
The flag is disabled by default.
* Fix test guard
* Fix formatting in the test
f08491eeca