[Tests] Add manual picture loader benchmark against the real card hosts (#7288)

* tests: add manual picture loader benchmark against the real card hosts

(cherry picked from commit 588d71d3b9a8278ab98b847802908f322c1035b7)

* tests: address review on picture loader benchmark

- Sandbox via unique app/org names plus Linux-only XDG redirection, derive
  the warm-cache probe and data path from SettingsCache, and bail out when the
  temporary sandbox cannot be created.
- Run each pass with a fresh worker and shut workers down before reading the
  429 counters, so the redirect cache is persisted and no worker thread can
  outlive the stack-local counters or the installed message handler.
- Make s_activeCounters atomic and always forward log output when the previous
  handler is the built-in (nullptr) one.
- Validate --timeout-min, scale the cached-pass budget with --count, honour the
  first --url as the stress template, and add the missing trailing newlines.
- Zero-initialise SettingsCache members so the benchmark mock cannot
  dereference an indeterminate pointer.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

tests/CMakeLists.txt

@@ -155,3 +155,9 @@ add_subdirectory(loading_from_clipboard)
 add_subdirectory(movecard_tests)
 add_subdirectory(oracle)
 add_subdirectory(settings)
+
+# picture_loader_benchmark links libcockatrice_settings, which only exists when
+# a client/UI-capable target is being built.
+if(WITH_ORACLE OR WITH_CLIENT)
+  add_subdirectory(picture_loader_benchmark)
+endif()