(KeepGoing) Add KEEP_GOING_CAUGHT_MIDEND_SIGNAL, --enable-ast-tests

* Add a new convenience macro to handle Midend signals.
  Note: This macro is intended to be used within a user's
  custom translator, surrounding their Midend logic.

* Add "--enable-ast-tests" to the tests/KeepGoingTranslator,
  while disabling the AST consistency tests by default now.
  This is intended to improve the performance of the translator
  for performance measurements.