support/testing/run-tests: help: put the one-letter form before the long form

support/testing/run-tests: help: put the one-letter form before the long form

This is what the manpages usually do, and what Python does with the
automatically-added -h/--help parameter:

Before the change:
  $ ./support/testing/run-tests
    [...]
  optional arguments:
    -h, --help            show this help message and exit
    --list, -l            list of available test cases
    --all, -a             execute all test cases

After the change:
  $ ./support/testing/run-tests
    [...]
  optional arguments:
    -h, --help            show this help message and exit
    -l, --list            list of available test cases
    -a, --all             execute all test cases

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>