Update meson config

[jeromekelleher]

We're getting some warnings from meson:

The Meson build system
Version: 0.56.0
Source dir: /home/jk/work/github/kastore/c
Build dir: /home/jk/work/github/kastore/c/build
Build type: native build
Project name: kastore
Project version: undefined
C compiler for the host machine: cc (gcc 9.3.0 "cc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0")
C linker for the host machine: cc ld.bfd 2.34
C++ compiler for the host machine: c++ (gcc 9.3.0 "c++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0")
C++ linker for the host machine: c++ ld.bfd 2.34
Host machine cpu family: x86_64
Host machine cpu: x86_64
meson.build:9: WARNING: Consider using the built-in warning_level option instead of using "-Wall".
meson.build:9: WARNING: Consider using the built-in warning_level option instead of using "-Wextra".
meson.build:9: WARNING: Consider using the built-in warning_level option instead of using "-Wpedantic".
Found pkg-config: /usr/bin/pkg-config (0.29.1)
Run-time dependency cunit found: YES 2.1-3
Build targets in project: 7

Might as well freshen up the config here for the 2.0.1 release.

We should make sure that this works with meson > 0.55, as this is what we've pinned to for tskit etc.

[jeromekelleher]

Also define the tests - running ninja -C build test is very handy