In old versions of rustc (1.15 through 1.29) it would cause a warning if
this #[macro_use] was not present.

    warning: proc macro crates and `#[no_link]` crates have no effect without `#[macro_use]`
       --> serde/src/lib.rs:340:1
        |
    340 | extern crate serde_derive;
        | ^^^^^^^^^^^^^^^^^^^^^^^^^^

These days serde_derive requires a newer compiler than that, so the
bogus warning would never occur.

This makes it slightly more convenient to use the following as a
Reindeer fixup for those that prefer to build from source:

    extra_mapped_srcs = { "src/lib_from_source.rs" = "src/lib.rs" }

    [platform_fixups.'cfg(all(target_arch = "x86_64", target_os = "linux", target_env = "gnu"))']
    extra_deps = [":proc-macro2", ":quote", ":syn"]

as opposed to checking in a whole new file containing the `extern crate
proc_macro` + `include!("lib_from_source.rs")`.

Delete unused serde_derive #[macro_use]

This has been superseded by inherent methods since Rust 1.26.

Move serde_test out to serde-rs/test

fix: don't check skipped variant with internal tag