This follows the interface proposed in
https://wiki.gnome.org/Initiatives/GnomeGoals/InstalledTests

 and can
be used for "as-installed" testing, which is a useful way to check for
regressions (for example if a GLib upgrade might cause a regression in
the already-compiled version of gcab), especially in frameworks like
Debian's autopkgtest and Fedora's Standard Test Interface.

To locate the test data in a way that is compatible with as-installed
tests, use g_test_build_filename() instead of hard-coding the source
directory into the test executable. This incidentally also avoids relying
on PATH_MAX, which is not completely portable.

Signed-off-by: Simon McVittie <smcv@debian.org>