test: Add coverage for enum/int conversions

Author: ryanofsky

test/mp/test/foo.capnp

@@ -27,6 +27,7 @@ interface FooInterface $Proxy.wrap("mp::test::FooImplementation") {
     passEmpty @12 (arg :FooEmpty) -> (result :FooEmpty);
     passMessage @13 (arg :FooMessage) -> (result :FooMessage);
     passMutable @14 (arg :FooMutable) -> (arg :FooMutable);
+    passEnum @15 (arg :Int32) -> (result :Int32);
 }
 
 interface FooCallback $Proxy.wrap("mp::test::FooCallback") {

test/mp/test/foo.h

@@ -21,6 +21,8 @@ struct FooStruct
     std::vector<bool> vbool;
 };
 
+enum class FooEnum : int { ONE = 1, TWO = 2, };
+
 struct FooCustom
 {
     std::string v1;
@@ -72,6 +74,7 @@ class FooImplementation
     FooEmpty passEmpty(FooEmpty foo) { return foo; }
     FooMessage passMessage(FooMessage foo) { foo.message += " call"; return foo; }
     void passMutable(FooMutable& foo) { foo.message += " call"; }
+    FooEnum passEnum(FooEnum foo) { return foo; }
     std::shared_ptr<FooCallback> m_callback;
 };