contrib/nitester: in verbose mode, each worker prints its completed task

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: skip long interpreter tests

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
contrib/nitester: tweak verbose level of some messages

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
contrib/nitester: sort tests per engine first

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>