Positive coverage: prove_syntactically_eq / symmetric

1 test exercised this rule:

Source location: crates/formality-rust/src/prove/test/eq_assumptions.rs:126

#[test]
fn test_normalize_assoc_ty_existential1() {
    test_prove(
        Program::empty(),
        term(
            "\
            forall<T> \
            exists<A> \
            { <T as Iterator>::Item = u32 } => { <A as Iterator>::Item = u32 }",
        ),
    )
    .assert_ok(expect!["{Constraints { env: Env { variables: [!ty_1, ?ty_2], bias: Soundness, pending: [], allow_pending_outlives: false }, known_true: true, substitution: {?ty_2 => !ty_1} }}"]);
}