support/testing/run-tests: fix --testcases option

The --testcases option of run-tests says how many test cases to build in
parallel. It automatically derives a jlevel from it by dividing the
number of cores + 1 by the number of parallel testcases. However, this
will typically result in a fractional number. Make doesn't like
fractional numbers as argument to -j.

Convert the number to integer (rounding down).

* br2_jlevel is an int, as multiprocessing.cpu_count() is an int, so it
  will be always >=2  (cpu_count() raises an error if it can't determine
  the number of CPU, so it will always return at least 1);

* args.testcases is an int, and is checked to be >=1

So br2_jlevel + args.testcases is guaranteed to always be bigger
than or equal to args.testcases, and the division thus bigger than 1.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[yann.morin.1998@free.fr:
  - ensure division provide at least 1
  - drop the test below
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>