tests: systemclock: Stop all stress threads before joining them

This reduces the chance of the main thread getting starved while trying
to shut down the test, potentially causing a timeout.

Even on an idle 96-processor system this reduces the duration of the
systemclock tests from ~8s to ~3s.

Part-of: <gstreamer/gstreamer!734>
37 jobs for !734 with systemclock-stress-shutdown in 33 minutes and 48 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #6655580
alpine amd64 manifest builder docker

00:00:30

passed #6655578
android docker

00:00:21

passed #6655577
cerbero fedora amd64 docker

00:00:31

passed #6655575
fedora amd64 docker

00:00:31

passed #6655581
gst-indent amd64 docker

00:00:31

 
  Preparation
passed #6655586
check allow-edit

00:00:18

passed #6655585
gst indent

00:00:26

passed #6655583
manifest

00:00:21

 
  Build
passed #6655603
gstreamer
build cerbero cross-android universal

00:13:01

passed #6655611
gst-ios-13.2
build cerbero cross-ios universal

00:22:45

passed #6655605
gstreamer
build cerbero cross win32

00:08:29

passed #6655607
gstreamer
build cerbero cross win64

00:06:51

passed #6655601
gstreamer
build cerbero fedora x86_64

00:07:05

passed #6655609
gst-macos-10.15
build cerbero macos x86_64

00:13:27

passed #6655613
docker windows gstreamer-windows 1809
build cerbero msvc x86_64

00:15:28

passed #6655593
build clang fedora x86_64

00:08:06

manual #6655599
docker windows 1809 allowed to fail manual
build msys2
passed #6655588
build nodebug fedora x86_64

00:07:39

passed #6655590
build static fedora x86_64

00:06:47

passed #6655591
build static nodebug fedora x86_64

00:07:27

passed #6655595
docker windows 1809
build vs2017 amd64

00:13:00

passed #6655597
docker windows 1809
build vs2017 x86

00:12:40

 
  Test
passed #6655616
gstreamer
check fedora

00:07:27

passed #6655618
gstreamer
integration testsuites fedora 1/4

00:15:11

passed #6655620
gstreamer
integration testsuites fedora 2/4

00:13:51

passed #6655622
gstreamer
integration testsuites fedora 3/4

00:13:17

passed #6655624
gstreamer
integration testsuites fedora 4/4

00:13:04

passed #6655629
gstreamer
valgrind bad

00:06:12

passed #6655626
gstreamer
valgrind base

00:07:15

passed #6655625
gstreamer
valgrind core

00:06:26

passed #6657057
gstreamer
valgrind ges

00:06:18

passed #6655627
gstreamer
valgrind good

00:07:37

passed #6655628
gstreamer
valgrind ugly

00:03:58

failed #6655630
gstreamer
valgrind ges

00:19:37

 
  Integrate
passed #6655631
cross-android universal examples

00:06:01

passed #6655633
gst-ios-13.2
cross-ios universal examples

00:03:42

passed #6655632
documentation

00:13:48