Reorganise the tests into module files, rather than all being in a
single file. No tests were changed in this patch.
The `serialize` flag was also not enabled in `test/Cargo.toml`. This
is enabled to ensure those tests are run moving forward.
Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett
