tests: kern: add some tests for TIOCSTI

These offer at least rudimentary coverage of TIOCSTI, ensuring that it
basically works and does what it's described to do and throws errors for
unprivileged u

tests: kern: add some tests for TIOCSTI

These offer at least rudimentary coverage of TIOCSTI, ensuring that it
basically works and does what it's described to do and throws errors for
unprivileged use that is supposed to be blocked.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D50507

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