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_as_str_variant/case.ww

@@ -0,0 +1,9 @@
+//ww:run-exit 3
+// Migrated from 700_e2e row 114.
+package main;
+fn fail() (i64 | str) = { return "bad"; };
+fn main() i32 = {
+  let r: (i64 | str) = fail();
+  let e: str = r as str;
+  return e.len: i32;
+};