meson: Build Qt5 tests with -std=c++11

We already do this for the plugin.

gstreamer/gst-plugins-base#780 (comment 548179)

Part-of: <gstreamer/gst-plugins-good!642>
26 jobs for !642 with fix-qt-tests-build-c++11 in 24 minutes and 28 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Preparation
passed #3297385
gst indent

00:00:17

passed #3297383
manifest

00:00:26

 
  Build
passed #3297400
gstreamer
build cerbero cross-android universal

00:09:58

passed #3297404
gst-ios-13.2
build cerbero cross-ios universal

00:22:28

passed #3297401
gstreamer
build cerbero cross win32

00:07:05

passed #3297402
gstreamer
build cerbero cross win64

00:05:57

passed #3297399
gstreamer
build cerbero fedora x86_64

00:04:34

passed #3297403
gst-macos-10.15
build cerbero macos x86_64

00:18:36

passed #3297392
build clang fedora x86_64

00:04:37

manual #3297398
1809 windows docker allowed to fail manual
build msys2
passed #3297387
build nodebug fedora x86_64

00:04:33

passed #3297388
build static fedora x86_64

00:04:26

passed #3297390
build static nodebug fedora x86_64

00:04:30

passed #3297393
1809 windows docker
build vs2017 amd64

00:10:35

passed #3297396
1809 windows docker
build vs2017 x86

00:10:12

 
  Test
passed #3297405
gstreamer
check fedora

00:06:14

passed #3297406
gstreamer
integration testsuites fedora 1/4

00:13:46

passed #3297407
gstreamer
integration testsuites fedora 2/4

00:13:53

passed #3297408
gstreamer
integration testsuites fedora 3/4

00:13:34

passed #3297409
gstreamer
integration testsuites fedora 4/4

00:15:50

passed #3297411
gstreamer
valgrind bad

00:04:16

passed #3297412
gstreamer
valgrind ges

00:05:28

passed #3297410
gstreamer
valgrind good

00:09:09

 
  Integrate
passed #3297413
cross-android universal examples

00:06:15

passed #3297415
gst-ios-13.2
cross-ios universal examples

00:01:26

passed #3297414
documentation

00:07:32