kunit: declare kunit_assert structs as const

Everywhere we use the assert structs now takes them via const*, as of
commit 7466886b ("kunit: take `kunit_assert` as `const`").

So now let's properly declare the structs as const as well.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>