tests: be a bit more strict cleaning up fifos

tests: be a bit more strict cleaning up fifos

When we re-factored we dropped the unlink() step which turns out to be
required for rmdir to do its thing. If we had been checking the return
value we would have noticed so lets do that with this fix.

Fixes: 68406d1085 (tests/unit: cleanups for test-io-channel-command)
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230228190653.1602033-4-alex.bennee@linaro.org>