bats's option parser translates "-j5" to "-j -5". This caused a negative
number of "slots" (threads) to be available and the test to silently
hang forever. Not nice.

Add a $num_jobs sanity check to fail immediately like this:

```
Invalid number of jobs: -5
tests.bats
   bats warning: Executed 0 instead of expected 3 tests

3 tests, 0 failures, 3 not run
```

Non-numeric arguments (e.g.: -jf) evaluate to zero which also fails the
check.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>