tests: fix compiling tests with beta and nightly toolchains

[rhysd]

CI is failing because memchr enables missing_docs lint but some exported macros in tests are not documented.

$ cargo +nightly test
   Compiling memchr v2.7.4 (/Users/rhysd/Develop/github.com/BurntSushi/memchr)
error: missing documentation for a macro
   --> src/tests/memchr/prop.rs:9:1
    |
9   | macro_rules! define_memchr_quickcheck {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
note: the lint level is defined here
   --> src/lib.rs:172:9
    |
172 | #![deny(missing_docs)]
    |         ^^^^^^^^^^^^

error: could not compile `memchr` (lib test) due to 1 previous error

This PR fixes the compile errors by allowing missing docs on the macros.