selftests: Add printf attribute to kselftest prints

Kselftest header defines multiple variadic functions that use printf
along with other logic.

There is no format checking for the variadic functions that use
printing inside kselftest.h. Because of this the compiler won't
be able to catch instances of mismatched printf formats and debugging
tests might be more difficult.

Add the common __printf() attribute macro to kselftest.h.

Add __printf() attribute to every function using formatted printing
with variadic arguments.

Adding the attribute and compiling all selftests exposes a number of
-Wformat warnings which were previously unnoticed due to a lack of
format specifiers checking by the compiler.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>