Unit tests for libparlatype, run with `make check`.

Valgrind test with `make check-valgrind-memcheck`.
Valgrind suppression files downloaded from:
https://cgit.freedesktop.org/gstreamer/common/tree/gst.supp
https://git.gnome.org/browse/glib/tree/glib.supp

Code coverage test with `make check-code-coverage`.
Compile lib with --enable-code-coverage first.
Result is in libparlatype-{VERSION}-coverage/index.html
