testsuite: add the req_ghc_with_threaded_rts predicate

This patch adds the req_ghc_with_threaded_rts predicate to the
testsuite to assert the platform has threaded RTS, and mark some tests
as req_ghc_with_threaded_rts. Also makes ghc_with_threaded_rts a
config field instead of a global variable.

(cherry picked from commit aa6afe8a)