Fix wiggle tests

crates/wiggle/tests/union.witx

@@ -5,27 +5,26 @@
 ;; Fight for the full product of your labor!
 
 (typename $reason
-  (union $excuse
-    (field $dog_ate f32)
-    (field $traffic s32)
-    (empty $sleeping)))
+  (variant (@witx tag $excuse)
+    (case $dog_ate f32)
+    (case $traffic s32)
+    (case $sleeping)))
 
 (typename $reason_mut
-  (union $excuse
-    (field $dog_ate (@witx pointer f32))
-    (field $traffic (@witx pointer s32))
-    (empty $sleeping)))
+  (variant (@witx tag $excuse)
+    (case $dog_ate (@witx pointer f32))
+    (case $traffic (@witx pointer s32))
+    (case $sleeping)))
 
 (module $union_example
   (@interface func (export "get_tag")
     (param $r $reason)
-    (result $error $errno)
-    (result $t $excuse)
+    (result $error (expected $excuse (error $errno)))
   )
 
   (@interface func (export "reason_mult")
     (param $r $reason_mut)
     (param $multiply_by u32)
-    (result $error $errno)
+    (result $error (expected (error $errno)))
   )
 )