Add focused txn_box comparison test

[bneradt]

Txn_box's documented tuple example was reported as failing to load, but
its existing coverage is embedded in a broad static-file test and does
not isolate either reported comparison form.

This adds a replay-based regression test that loads the default txn_box
root as a global plugin and verifies heterogeneous tuple selection and
scalar status selection through response mutations.

Fixes: #12335

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[bryancall]

Approving. I built this locally with experimental plugins on and ran the test, and
confirmed it's doing real work rather than passing vacuously: it runs (Skipped: 0,
so the PluginExists gate isn't quietly skipping it), and it fails as expected both
when the tuple is made unmatchable (match: "example" -> "NOMATCH") and when the
config is given a parse error.

One thing that might let us close #12335 on firmer ground. The tuple form here was
already covered and passing at the 10.2.0 tag itself --
tests/gold_tests/pluginTest/txn_box/static_file/static_file.replay.yaml has the
identical with: [ upstream-rsp-status , proxy-req-path ] + as-tuple block, and
that's the snippet doc/admin-guide/plugins/txn_box/examples.en.rst literal-includes
as the documented example. No comparison-parsing changes have landed in txn_box since
10.2.0 either (that window is Coverity/ASAN/OpenSSL cleanups). So instead of "it works
on master," I think we can tell the reporter something stronger: the documented form
wasn't broken on the version they reported either, so the difference has to be in
their actual config or environment -- and we'd need the real file to get further.
Might be worth reopening just long enough to ask for it.

Two smaller notes:

• The description says the existing coverage "does not isolate either reported
  comparison form." The tuple form is isolated (and doc-linked) in static_file. What's
  genuinely new here is worth calling out instead: this loads a standalone
  txn_box.yaml with the default txn_box: root key via plugin.config, whereas
  static_file goes through TxnBoxTestAndRun with config_key="meta.txn_box.global".
  The standalone path is what an operator actually uses, and only
  prod/mTLS.txnbox.yaml otherwise touches it -- that's a good argument for this test
  existing. The scalar select case is new too.

• The reporter's second snippet had no do: under - eq: 200:

  - with: upstream-rsp-status
    select:
    - eq: 200

  This test adds one, so that literal form is still unexercised. Probably not the
  cause, but worth knowing it's untested if the issue comes back.