Macro encoding_index_tests::multi_byte_tests

macro_rules! multi_byte_tests {
    (make shared tests and benches with dups = $dups:expr) => { ... };
    (
        mod = $parentmod:ident, // XXX Rust issue #20701
        dups = $dups:expr
    ) => { ... };
    (
        mod = $parentmod:ident, // XXX Rust issue #20701
        remap = [$remap_min:expr, $remap_max:expr],
        dups = $dups:expr
    ) => { ... };
}

Makes a common test suite for multi-byte indices.