Comine integration tests into one binary (#951)

Signed-off-by: Nico Burns <nico@nicoburns.com>

tests/hand_written.rs

@@ -0,0 +1,10 @@
+mod hand_written {
+    mod border_and_padding;
+    mod caching;
+    mod measure;
+    mod min_max_overrides;
+    mod relayout;
+    mod root_constraints;
+    mod rounding;
+    mod serde;
+}

tests/xml.rs

@@ -6,6 +6,7 @@ use taffy::{
 };
 use taffy_test_helpers::{test_measure_function, TestNodeContext};
 
+#[path = "./xml/mod.rs"]
 mod xml;
 
 #[derive(Debug)]