kunit: Annotate _MSG assertion variants with gnu printf specifiers

KUnit's assertion macros have variants which accept a printf format
string, to allow tests to specify a more detailed message on failure.
These (and the related KUNIT_FAIL() macro) ultimately wrap the
__kunit_do_failed_assertion() function, which accepted a printf format
specifier, but did not have the __printf attribute, so gcc couldn't warn
on incorrect agruments.

It turns out there were quite a few tests with such incorrect arguments.

Add the __printf() specifier now that we've fixed these errors, to
prevent them from recurring.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>