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_for_sum/case.ww

@@ -0,0 +1,9 @@
+//ww:run-exit 45
+// Migrated from 700_e2e row 2.
+package main;
+fn main() i32 = {
+  let i: i32 = 0;
+  let s: i32 = 0;
+  for (i < 10) { s += i; i += 1; };
+  return s;
+};