test/stubgen: add interface gen test

tests/schemas/multi/base.capnp

@@ -0,0 +1,11 @@
+@0xa1a2a3a4a5a6a7a8;
+
+struct Person {
+  name @0 :Text;
+  age @1 :Int32;
+}
+
+enum Status {
+  active @0;
+  inactive @1;
+}

tests/schemas/multi/consumer.capnp

@@ -0,0 +1,8 @@
+@0xb1b2b3b4b5b6b7b8;
+
+using Import = import "base.capnp";
+
+struct Team {
+  members @0 :List(Import.Person);
+  status @1 :Import.Status;
+}

tests/schemas/test_interface.capnp

@@ -0,0 +1,11 @@
+@0xa1b2c3d4e5f6a7b8;
+
+interface Calculator {
+  evaluate @0 (expression :Text) -> (value :Float64);
+  add @1 (a :Float64, b :Float64) -> (result :Float64);
+  ping @2 () -> ();
+}
+
+struct CalcServer {
+  calc @0 :Calculator;
+}