Add cheribsdtest tests relating to tag stripping in userspace and also by system calls

[rwatson]

In order to better test QEMU userlevel support for CheriABI, it would be useful to have a set of tests around tag stripping -- both simply in userlevel code, but also in terms of ensuring that kernel access to user memory strips tags properly, such as copyout()'s from system calls, but also for things like signal frames written to stacks. Tests should cover things like partial overwrites of capabilities, but also full replacement with both valid and invalid capabilities.

CC @kwitaszczyk, @brooksdavis