kunit: Fix the comment of KUNIT_ASSERT_STRNEQ as assertion

The current comment for KUNIT_ASSERT_STRNEQ incorrectly describes it as
an expectation. Since KUNIT_ASSERT_STRNEQ is an assertion, updates the
comment to correctly refer to it as such.

Signed-off-by: Eric Chan <ericchancf@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>