Enable disjoint union tests

Summary:
["Files in subdirectories of the tests directory don’t get compiled as separate crates or have sections in the test output."](https://doc.rust-lang.org/book/ch11-03-test-organization.html), so the `test/datatype/disjoint_union_test.rs` test is not run by the rust workflow.

These changes move `test/datatype/disjoint_union_test.rs` to `test/disjoint_union_test.rs` and add `#[allow(dead_code)]`s to avoid warnings generated by this test.

The rust workflow should show some additional test results:

```
Running `/home/runner/work/SPARTA/SPARTA/rust/target/debug/deps/disjoint_union_test-7d8dce3f7696edda`

running 6 tests
test test_basic_union_top_bottom_leq ... ok
test test_diff_arms_no_leq ... ok
test test_join_same_arm ... ok
test test_join_diff_arm ... ok
test test_meet_diff_arm ... ok
test test_meet_same_arm ... ok
```

X-link: facebook/SPARTA#26

Reviewed By: arnaudvenet

Differential Revision: D64787297

Pulled By: yuxuanchen1997

fbshipit-source-id: 3175d2a23e9ae34806cafd9c97754709f2ab189c

sparta/rust/tests/datatype/disjoint_union_test.rs → sparta/rust/tests/disjoint_union_test.rs

@@ -11,6 +11,7 @@ use sparta::datatype::AbstractDomain;
 use sparta::datatype::DisjointUnion;
 use sparta::datatype::HashSetAbstractDomain;
 
+#[allow(dead_code)]
 #[derive(Clone, DisjointUnion, PartialEq, Eq)]
 enum MyUnionedDomain {
     FirstCase(HashSetAbstractDomain<i32>),
@@ -119,6 +120,7 @@ fn test_meet_diff_arm() {
     assert!(met_mudom.is_bottom());
 }
 
+#[allow(dead_code)]
 #[derive(Clone, DisjointUnion, PartialEq, Eq)]
 enum TestGenericsDeriveTypechecks<S, T>
 where
@@ -129,6 +131,7 @@ where
     SecondCase(HashSetAbstractDomain<T>),
 }
 
+#[allow(dead_code)]
 #[derive(Clone, DisjointUnion, PartialEq, Eq)]
 enum TestGenericsDeriveForWholeDomainTypechecks<S: AbstractDomain, T>
 where