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_append_spread/case.ww

@@ -0,0 +1,15 @@
+//ww:run-exit 3
+// Migrated from 700_e2e row 41.
+package main;
+import os;
+fn main() i32 = {
+  let src: []u8;
+  src.ptr = nil; src.len = 0; src.cap = 0;
+  append(src, 65u8, 66u8, 67u8);
+  let dst: []u8;
+  dst.ptr = nil; dst.len = 0; dst.cap = 0;
+  append(dst, src...);
+  os.write(1, dst.ptr, dst.len: u64);
+  os.write(1, "\n".ptr, 1u64);
+  return dst.len;
+};