test: Add a `make install` target for the tests This makes it more convenient to compile them alongside SDL, install them in an optional package and use them as smoke-tests or diagnostic tools. The default installation directory is taken from GNOME's installed-tests, which seems as good a convention as any other: https://wiki.gnome.org/Initiatives/GnomeGoals/InstalledTests Signed-off-by: Simon McVittie <smcv@collabora.com>