exterr(9): add a few tests of new message formats

[brooks]

Summary

This is enough to show that the idea works (and to exercise
uexterr_set()), but isn't complete by any means.

Effort: CHERI upstreaming
Sponsored by: Innovate UK

[brooks]

Use UEXTERROR().

[brooks]

Update for UEXTERROR returning void.

[kib]

tests/sys/kern/exterr_test.c

97

I am curious, what is the point of printing something out from ATF tests? Is it usable in some way?

[brooks]

tests/sys/kern/exterr_test.c

97

When the tests fail the output ends up in the verbose reports and I've found it fairly useful when debugging these changes, especially in qemu VM where I don't have a compiler installed and don't want to build things by hand.

That being said, the simple strcmp below should be a ATF_CHECK_STREQ which has the same effect.

[brooks]

Improve test description