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_try_str_prop/case.ww

@@ -0,0 +1,19 @@
+//ww:run-exit 4
+// Migrated from 700_e2e row 60.
+package main;
+fn try1(n: i64) (i64 | str) = {
+  if (n < 0) { return "fail"; };
+  return n;
+};
+fn try2(n: i64) (i64 | str) = {
+  let v: i64 = try1(n)?;
+  return v + 100;
+};
+fn main() i32 = {
+  let r: (i64 | str) = try2(-1);
+  match (r) {
+    case let n: i64 => return n: i32;
+    case let s: str => return s.len: i32;
+  };
+  return 0;
+};