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_tuple_str_positional/case.ww

@@ -0,0 +1,8 @@
+//ww:run-exit 47
+// Migrated from 700_e2e row 107.
+package main;
+fn split() (i64, str) = { return 42, "hello"; };
+fn main() i32 = {
+  let t: (i64, str) = split();
+  return (t.0: i32) + (t.1.len: i32);
+};