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_variadic_gather/case.ww

@@ -0,0 +1,12 @@
+//ww:run-exit 42
+// Migrated from 700_e2e row 128.
+package main;
+fn sum(args: i64...) i64 = {
+  let s: i64 = 0i64;
+  let i: i32 = 0;
+  for (i < args.len) { s += args[i]; i += 1; };
+  return s;
+};
+fn main() i32 = {
+  return sum(1i64, 2i64, 3i64, 7i64, 9i64, 20i64): i32;
+};