add simple test for SingleSegmnetAllocator

capnp/tests/single_segment_allocator.rs

@@ -0,0 +1,13 @@
+use capnp::message;
+
+#[test]
+pub fn single_segment_allocator() {
+    let mut buffer = capnp::Word::allocate_zeroed_vec(200);
+    let allocator =
+        message::SingleSegmentAllocator::new(capnp::Word::words_to_bytes_mut(&mut buffer[..]));
+    let mut msg = message::Builder::new(allocator);
+    msg.set_root("hello world!").unwrap();
+
+    let s: capnp::text::Reader = msg.get_root_as_reader().unwrap();
+    assert_eq!("hello world!", s);
+}