Round-trip encoding/decoding of many types is tested in
`compiler/rustc_serialize/tests/opaque.rs`. There is also a small amount
of encoding/decoding testing in three files in `tests/ui-fulldeps`.
There is no obvious reason why these three files are necessary. They
were originally added in 2014. Maybe it wasn't possible for a proc
macro to run in a unit test back then?
This commit just moves the testing from those three files into the unit
test.
8d359e4385