Add derive coverage for big-endian field representations

[daixihegu]

This PR adds test coverage for #[derive(PrimeField)] when using
#[PrimeFieldReprEndianness = "big"].

The existing derive tests exercise little-endian representations, but the derive
macro also supports big-endian representations. The new test defines a small
prime field, checks that to_repr emits the expected big-endian bytes, and
verifies that both from_repr and from_repr_vartime round-trip correctly.

I have test this change by executing:

• cargo test --all-features
• cargo test --no-default-features --features derive
• cargo fmt -- --check