Revive the "long suite" and run it on master merges

We used to run a separate "long suite" of additional tests not on every commit but periodically. Today that suite is not run and is likely bit-rotted: xref #1807 about failures in the long suite. Also xref #1793: clean up nightly suite and remove no longer supported configurations.

The suggestion here is to revive the long suite, remove no-longer-supported configurations and tests (overlaps with #1793), and run it on every master merge. This would pull in important tests like api.ibl-stress-aarch64-far-link_LONG which today are not run at all.

We are already running a shorter suite on pull requests than master merges, from #4059 (closed) and in particular PR #4079, which uses this code:

if (WIN32 AND NOT X64 AND "$ENV{CI_TRIGGER}" STREQUAL "pull_request")
  set(SKIP_LESS_DIFFERENTIATED_TESTS_FOR_PULL_REQUEST ON)
else ()
  set(SKIP_LESS_DIFFERENTIATED_TESTS_FOR_PULL_REQUEST OFF)
endif ()

We can use the same CI_TRIGGER env var set by our GA workflows to enable TEST_LONG.