buildutil: Import some test helpers from json-glib

Not actually used yet; we'll need to modify these in subsequent
commits, but this is the starting point.  Future commits will
therefore have a useful diff.  Even if admittedly it's unlikely
they'll re-unify in the near future.
tests: Enable glib-tap.mk

These files were taken from json-glib, around the era of this commit:

https://git.gnome.org/browse/json-glib/tree/build/autotools?id=2779d537492f1902d71cf648631238110b62b311

Unfortunately, this involved hacking it up a bit:
 - I couldn't easily use `nobase` for the data, so I deleted that.
   Test data goes in the installed-tests dir.
 - Delete duplicated predeclared variables; we're using nonrecursive
   make.
 - Ensure we run each test in its own tmpdir
tests/jsonutil: New test

Adding some basic coverage of the json parsing.