build: Rework conditional support for tests and benchmarks

The default should be to continue the configuration process if no GLib
was found, but fail if either (or both) tests and benchmarks were
explicitly enabled and GLib is not available.
tests: Add pre-seeded Point variables

And rename the pre-seeded Point3D variables to avoid conflicts.
tests/point: Port to the boilerplate macros
tests/quad: Port to the boilerplate macros
tests/quaternion: Port to the boilerplate macros
tests/rect: Port to the boilerplate macros