skip only fork-based tests, and do that when DBD_FIREBIRD_TEST_SKIP_EVENTS_FORK is present in the environment

DBD_FIREBIRD_TEST_SKIP_EVENTS support dropped. thread-based event tests
are now skipped in AUTOMATED_TESTING environments, see d373415

fork-based tests still fail on kfreebsd, probably because of a bug in
semaphore synchronisation stuff in firebird/embedded (parent tries to
operate on a semaphore already destroyed by the child)
release 1.14