Sporadic test failures

We're starting to see new regular sporadic test failures. Some of them seem to be a complete test timeout, with no logs whatsoever.

I managed to reproduce one locally, with RUN_TEST( Tests, ReplaceVlcInstance ); and no_album_artist

Thread 8 (Thread 0x7f9e06ccb700 (LWP 122717) "samples"):
#0  __lll_lock_wait (futex=futex@entry=0x555582d0d168, private=0) at lowlevellock.c:52
#1  0x00007f9e08d3f843 in __GI___pthread_mutex_lock (mutex=0x555582d0d168) at ../nptl/pthread_mutex_lock.c:80
#2  0x0000555580db9824 in __gthread_mutex_lock (__mutex=0x555582d0d168) at /usr/include/x86_64-linux-gnu/c++/10/bits/gthr-default.h:749
#3  0x0000555580db9b0c in std::mutex::lock (this=0x555582d0d168) at /usr/include/c++/10/bits/std_mutex.h:100
#4  0x0000555580dbc750 in std::lock_guard<std::mutex>::lock_guard (this=0x7f9e06ccac48, __m=...) at /usr/include/c++/10/bits/std_mutex.h:159
#5  0x00007f9e095ecb3f in medialibrary::MediaLibrary::startFsFactoriesAndRefresh (this=0x555582d0d160) at ../src/MediaLibrary.cpp:2722
#6  0x00007f9e096772f7 in medialibrary::DiscovererWorker::runReloadAllDevices (this=0x555582eed870) at ../src/discoverer/DiscovererWorker.cpp:577
#7  0x00007f9e0967647f in medialibrary::DiscovererWorker::run (this=0x555582eed870) at ../src/discoverer/DiscovererWorker.cpp:389
#8  0x00007f9e09682525 in std::__invoke_impl<void, void (medialibrary::DiscovererWorker::*)(), medialibrary::DiscovererWorker*> (__f=@0x555582ed29d0: (void (medialibrary::DiscovererWorker::*)(medialibrary::DiscovererWorker * const)) 0x7f9e096763f4 <medialibrary::DiscovererWorker::run()>, __t=@0x555582ed29c8: 0x555582eed870) at /usr/include/c++/10/bits/invoke.h:73
#9  0x00007f9e09682467 in std::__invoke<void (medialibrary::DiscovererWorker::*)(), medialibrary::DiscovererWorker*> (__fn=@0x555582ed29d0: (void (medialibrary::DiscovererWorker::*)(medialibrary::DiscovererWorker * const)) 0x7f9e096763f4 <medialibrary::DiscovererWorker::run()>) at /usr/include/c++/10/bits/invoke.h:95
#10 0x00007f9e096823d7 in std::thread::_Invoker<std::tuple<void (medialibrary::DiscovererWorker::*)(), medialibrary::DiscovererWorker*> >::_M_invoke<0ul, 1ul> (this=0x555582ed29c8) at /usr/include/c++/10/thread:264
#11 0x00007f9e09682390 in std::thread::_Invoker<std::tuple<void (medialibrary::DiscovererWorker::*)(), medialibrary::DiscovererWorker*> >::operator() (this=0x555582ed29c8) at /usr/include/c++/10/thread:271
#12 0x00007f9e09682374 in std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (medialibrary::DiscovererWorker::*)(), medialibrary::DiscovererWorker*> > >::_M_run (this=0x555582ed29c0) at /usr/include/c++/10/thread:215
#13 0x00007f9e08e3eed0 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#14 0x00007f9e08d3cea7 in start_thread (arg=<optimized out>) at pthread_create.c:477
#15 0x00007f9e08c6adef in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7f9e084f4800 (LWP 122611) "samples"):
#0  __pthread_clockjoin_ex (threadid=140316695639808, thread_return=0x0, clockid=<optimized out>, abstime=<optimized out>, block=<optimized out>) at pthread_join_common.c:145
#1  0x00007f9e08e3f0e3 in std::thread::join() () from /lib/x86_64-linux-gnu/libstdc++.so.6
#2  0x00007f9e09675343 in medialibrary::DiscovererWorker::stop (this=0x555582eed870) at ../src/discoverer/DiscovererWorker.cpp:79
#3  0x00007f9e095edb0f in medialibrary::MediaLibrary::setExternalLibvlcInstance (this=0x555582d0d160, inst=0x555582ee2c60) at ../src/MediaLibrary.cpp:2965
#4  0x0000555580da3798 in ReplaceVlcInstance (T=0x555582d0ce20) at ../test/samples/main.cpp:114
#5  0x0000555580da5c5d in main (ac=3, av=0x7ffdd919ebf8) at ../test/samples/main.cpp:239