Improve test framework: fix job control bugs

The non-deterministic bug of "job %N terminated without result" is now fixed.
The cause was that Bash job numbers are allocated as max(current job numbers) +
1, so if the most recent job exited was not harvested before the next job was
started, then the job number would get re-used, deleting the un-harvested job's
result file.

SIGINT (ctrl-C interrupt) handling has been improved for the -j (parallel) and
non-parallel cases.

Test case execution in the non-parallel case is faster, by fixing a bug that
waited for a one-second timeout before starting each test job.