test: migrate 700_e2e rows to corpus fixtures; retire the carrier

136 rows migrated, 0 already owned, 3 wwstage-failing rows held out.

test/wcc/data/r700_alias_union_arg/case.ww

@@ -0,0 +1,16 @@
+//ww:run-exit 107
+// Migrated from 700_e2e row 73.
+package main;
+type result = (i64 | i32);
+fn unwrap(r: result) i64 = {
+  match (r) {
+    case let v: i64 => return v;
+    case let e: i32 => return e: i64;
+  };
+  return -1;
+};
+fn main() i32 = {
+  let r1: result = 100;
+  let r2: result = 7: i32;
+  return (unwrap(r1) + unwrap(r2)): i32;
+};