Feature: build: Add support for glib-based unit tests.

This is largely, but not entirely, documented in the GLib docs:

https://developer.gnome.org/glib/unstable/glib-Testing.html#glib-Testing.description

The following changes are needed to make "make check" build and run
unit tests placed in the tests/ directory:

• Add mk/glib-tap.mk. This makefile chunk will be included in tests/Makefile.am and interfaces between the automake TAP harness and glib. The glib docs encourage this to be added.

• Add tests/tap-driver.sh and tests/tap-test. These are also part of the automake TAP harness and are also encouraged by the glib docs.

• Add m4/glibtests.m4. This sets up several macros that glib-tap.mk assumes will exist and adds a couple ./configure options that we probably won't care about, but other people may assume are present. This then requires GLIB_TESTS to be called from configure.ac.