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_alloc_slice_str/case.ww

@@ -0,0 +1,8 @@
+//ww:run-exit 4
+// Migrated from 700_e2e row 38.
+package main;
+import rt;
+fn main() i32 = {
+  let s: []str = alloc([], 4)!;
+  return s.cap;
+};