ci: gate testcore stress test (#1913)

Keep testcore built while excluding it from default make check runs.
Add --enable-testcore and enable it on master or via force_testcore/force_long_tests PR labels.
Handle label-triggered workflow runs without cancelling normal CI.

Related to #1809

Assisted-by: codex:gpt-5.6-sol
Assisted-by: claude-code:claude-fable-5