refactor/tests: fix and refactor tests

Fix tests to work with new memory management API. Refactor functions
for writing data to tensors and checking their contents. Merge set up
stage and compute/verify stage: it's much nicer to see input and output
values in one place. Refactor tests to be generic on backend and use
macros to eliminate repetitive test definitions.

src/frameworks/native.rs

@@ -254,7 +254,7 @@ mod test {
     #[test]
     fn it_converts_correctly_to_and_from_matrix() {
         let backend = get_native_backend();
-        let mut a = SharedTensor::<f32>::new(&vec![3, 2]).unwrap();
+        let mut a = SharedTensor::<f32>::new(&vec![3, 2]);
         write_to_memory(a.write_only(backend.device()).unwrap(),
             &[2f32, 5f32,
               2f32, 5f32,