sigreturn(2), longjmp(3) out of a signal handler lacks a test cases in cheribsdtest

[rwatson]

Currently we don’t appear to have a test case for direct return from a signal handler via sigreturn(2) in cheribsdtest. This gap came up in a discussion about library compartmentalisation and the need to support longjmp(3) out of signal handlers, which also likely wants a test case of its own.

(Discussed with @dpgao.)