kunit: take `kunit_assert` as `const`

The `kunit_do_failed_assertion` function passes its
`struct kunit_assert` argument to `kunit_fail`. This one,
in turn, calls its `format` field passing the assert again
as a `const` pointer.

Therefore, the whole chain may be made `const`.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>