meson: use a feature option for tests instead of boolean This allows tests to be autodetected gracefully, which is particularly convenient for kyua and atf_sh which are fairly painful to build and install by hand. Those who want to ensure tests are enabled or disabled may pass `-Dtests=enabled` or `-Dtests=disabled` respectively. This does require a modest bump in the required meson version to 0.49, which was released at the end of 2018, so roughly 4 years ago.