validate:launcher: Ensure a positive job count.

The default number of jobs to use is half of the available cores
rounded down, but in situations where only one core is available (such
as under some VMs), this means that `gst-validate-launcher` defaults
to using zero jobs, a case that the test-running code is not prepared
to handle.

This change corrects the documentation for the `--jobs` option, guards
against negative values both in the default setting and in argument
parsing, and introduces some defensive programming to prevent other
situations where the code might try to use zero jobs.  It also
modernizes the flooring division to use Python 3's `//` operator.
30 jobs for ensure_positive_job_count in 64 minutes and 20 seconds (queued for 19 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #1426023
gst indent

00:00:24

passed #1426022
manifest

00:00:47

 
  Build
passed #1426033
gstreamer
build cerbero cross-android universal

00:10:52

passed #1426037
gst-ios-13.2
build cerbero cross-ios universal

00:25:26

passed #1426034
gstreamer
build cerbero cross win32

00:07:17

passed #1426035
gstreamer
build cerbero cross win64

00:04:48

passed #1426032
gstreamer
build cerbero fedora x86_64

00:06:46

passed #1426036
gst-macos-10.15
build cerbero macos x86_64

00:12:04

passed #1426028
build clang fedora x86_64

00:06:34

passed #1426024
build fedora x86_64

00:08:29

passed #1426031
docker windows 1809
build msys2

00:33:41

passed #1426025
build nodebug fedora x86_64

00:03:25

passed #1426026
build static fedora x86_64

00:18:09

passed #1426027
build static nodebug fedora x86_64

00:02:57

passed #1426029
docker windows 1809
build vs2017 amd64

00:13:44

passed #1426030
docker windows 1809
build vs2017 x86

00:13:31

 
  Test
passed #1426038
gstreamer
check fedora

00:04:09

passed #1426039
gstreamer
integration testsuites fedora 1/4

00:13:54

passed #1426040
gstreamer
integration testsuites fedora 2/4

00:15:01

passed #1426041
gstreamer
integration testsuites fedora 3/4

00:12:24

passed #1426042
gstreamer
integration testsuites fedora 4/4

00:13:59

passed #1426047
gstreamer
local valgrind bad

00:07:05

passed #1426044
gstreamer
local valgrind base

00:14:08

passed #1426043
gstreamer
local valgrind core

00:09:02

passed #1426048
gstreamer
local valgrind ges

00:06:53

passed #1426045
gstreamer
local valgrind good

00:08:50

passed #1426046
gstreamer
local valgrind ugly

00:02:01

 
  Integrate
passed #1426049
cross-android universal examples

00:06:23

passed #1426051
gst-ios-13.2
cross-ios universal examples

00:04:17

passed #1426050
documentation

00:05:16