tests/tcg/xtensa: clean up test set

tests/tcg/xtensa: clean up test set

Drop test_fail: we know that exit simcall works. Now that it's not run
automatically there's no point in keeping it.
Drop test_pipeline: we're not modeling pipeline, we don't control ccount
and there's no plan to do so.
Enable test_boolean: it won't break on cores without boolean option, it
will do testing on cores with boolean option.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>