MySQL tests are now enabled by default as they no longer need extra
setup to be run.

Before that, you had to have a running instance of mysqld on your
machine to be able to run those tests. Now, provided you have mysqld
installed, you can just run the mysql tests like any other.

Introduces a suite of type-safe abstractions to interact with POSIX
processes and pipes. Due to insufficient support for C++17 on Debian 9,
these abstractions currently only live in the tester/ directory, behind
the ENABLE_UNIT_TESTS_MYSQL CMake flag.

(cherry picked from commit f1aa741d)