mini-os: put sanity_check() under CONFIG_TEST

mini-os: put sanity_check() under CONFIG_TEST

Hide the sanity_check() function, as it is used nowhere. By putting it
under #ifdef CONFIG_TEST it will stay around, but it won't be
included in normal production builds.

Call sanity_check() from the periodic thread of the test app, causing
a sanity check every second.

Since any application linked with Mini-OS can't call sanity_check()
(there is no EXPORT_SYMBOL for it), there is zero chance of breaking
any use case.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>