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_sum_param_str_rune/case.ww

@@ -0,0 +1,14 @@
+//ww:run-exit 42
+// Migrated from 700_e2e row 124.
+package main;
+fn pick(n: (str | rune)) i32 = {
+  match (n) {
+    case let s: str  => return s.len + 100;
+    case let r: rune => return r: i32;
+  };
+};
+fn main() i32 = {
+  let a: i32 = pick("hi");
+  let b: i32 = pick('?');
+  return a + b - 123;
+};