test: Add passDouble smoke test

test/mp/test/foo.capnp

@@ -30,6 +30,7 @@ interface FooInterface $Proxy.wrap("mp::test::FooImplementation") {
     passMessage @13 (arg :FooMessage) -> (result :FooMessage);
     passMutable @14 (arg :FooMutable) -> (arg :FooMutable);
     passEnum @15 (arg :Int32) -> (result :Int32);
+    passDouble @23 (arg :Float64) -> (result :Float64);
     passFn @16 (context :Proxy.Context, fn :FooFn) -> (result :Int32);
     callFn @17 () -> ();
     callFnAsync @18 (context :Proxy.Context) -> ();

test/mp/test/foo.h

@@ -86,6 +86,7 @@ class FooImplementation
     FooMessage passMessage(FooMessage foo) { foo.message += " call"; return foo; }
     void passMutable(FooMutable& foo) { foo.message += " call"; }
     FooEnum passEnum(FooEnum foo) { return foo; }
+    double passDouble(double value) { return value; }
     int passFn(std::function<int()> fn) { return fn(); }
     std::vector<FooDataRef> passDataPointers(std::vector<FooDataRef> values) { return values; }
     std::shared_ptr<FooCallback> m_callback;

test/mp/test/test.cpp

@@ -230,6 +230,8 @@ KJ_TEST("Call FooInterface methods")
     foo->passMutable(mut);
     KJ_EXPECT(mut.message == "init build pass call return read");
 
+    KJ_EXPECT(foo->passDouble(1.25) == 1.25);
+
     KJ_EXPECT(foo->passFn([]{ return 10; }) == 10);
 
     std::vector<FooDataRef> data_in;