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_match_yield_str/case.ww

@@ -0,0 +1,15 @@
+//ww:run-exit 103
+// Migrated from 700_e2e row 65.
+package main;
+fn pick(b: bool) (i32 | str) = {
+  if (b) { return 7; };
+  return "abc";
+};
+fn main() i32 = {
+  let r: (i32 | str) = pick(false);
+  let v: i32 = match (r) {
+    case let n: i32 => yield n + 1;
+    case let s: str => yield s.len: i32 + 100;
+  };
+  return v;
+};