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_vtable_fnptr/case.ww

@@ -0,0 +1,9 @@
+//ww:run-exit 42
+// Migrated from 700_e2e row 29.
+package main;
+type ops = struct { add: fn(a: i32, b: i32) i32 };
+fn plus(a: i32, b: i32) i32 = { return a + b; };
+fn main() i32 = {
+  let v: ops = ops { add = plus };
+  return v.add(20, 22);
+};