Rosetta testing breaks 'cargo test'

[LingMan]

Since #148 cargo test fails to compile.

$ cargo test
   Compiling naga v0.2.0 (git/naga)
error[E0277]: the trait bound `Module: serde::ser::Serialize` is not satisfied
  --> src/front/wgsl_rosetta_tests.rs:13:18
   |
13 |     let output = ron::ser::to_string_pretty(&module, Default::default()).unwrap();
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `serde::ser::Serialize` is not implemented for `Module`
   | 
  ::: .cargo/registry/src/github.com-1ecc6299db9ec823/ron-0.5.1/src/ser/mod.rs:30:8
   |
30 |     T: Serialize,
   |        --------- required by this bound in `ron::ser::to_string_pretty`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `naga`.

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed

The problem is that the serialize feature needs to be enabled. That's also the reason this wasn't caught in CI since --all-features is enabled there.

cc @pjoe

[kvark]

We should gate these tests by feature = "serialize"

[pjoe]

Right, guess it should be a matter of adding:

#[cfg(feature="serialize")]

a couple of strategic places :)