Ignore needless_pass_by_ref_mut clippy lint in test suite

    warning: this argument is a mutable reference, but not used mutably
       --> tests/ffi/lib.rs:524:34
        |
    524 | fn r_return_mut_rust_vec(shared: &mut ffi::Shared) -> &mut Vec<u8> {
        |                                  ^^^^^^^^^^^^^^^^ help: consider changing to: `&ffi::Shared`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
        = note: `-W clippy::needless-pass-by-ref-mut` implied by `-W clippy::all`

    warning: this argument is a mutable reference, but not used mutably
       --> tests/test.rs:222:24
        |
    222 |     fn callback_mut(s: &mut String) {
        |                        ^^^^^^^^^^^ help: consider changing to: `&String`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
        = note: `-W clippy::needless-pass-by-ref-mut` implied by `-W clippy::all`

tests/ffi/lib.rs

@@ -6,6 +6,7 @@
     clippy::missing_safety_doc,
     clippy::must_use_candidate,
     clippy::needless_lifetimes,
+    clippy::needless_pass_by_ref_mut,
     clippy::needless_pass_by_value,
     clippy::ptr_arg,
     clippy::trivially_copy_pass_by_ref,

tests/test.rs

@@ -5,6 +5,7 @@
     clippy::cast_possible_wrap,
     clippy::float_cmp,
     clippy::needless_pass_by_value,
+    clippy::needless_pass_by_ref_mut,
     clippy::unit_cmp,
     clippy::unseparated_literal_suffix
 )]