check: Add valgrind leak exception for srt test

From !6080 (closed)

cc @jonas.danielsson

check if it's environment related or something in the commit message or so..

---

The srt unittest test_src_listener_sink_call will sometimes fail under valgrind with the following splat:

 Memcheck, a memory error detector
 Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
 Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
 Parent PID: 14579

 HEAP SUMMARY:
     in use at exit: 799,848 bytes in 2,182 blocks
   total heap usage: 64,090 allocs, 61,908 frees, 37,891,032 bytes allocated

 120 bytes in 1 blocks are definitely lost in loss record 1,563 of 1,681
    at 0x4842FF5: operator new(unsigned long) (vg_replace_malloc.c:422)
    by 0x6031E29: srt::sync::SetThreadLocalError(CUDTException const&) (sync_posix.cpp:461)
    by 0x5FCD77E: CUDT::epoll_wait(int, std::set<int, std::less<int>,
    std::allocator<int> >*, std::set<int, std::less<int>,
    std::allocator<int> >*, long, std::set<int, std::less<int>,
    std::allocator<int> >*, std::set<int, std::less<int>, std::allocator<int> >*) [clone .cold] (api.cpp:3796)
    by 0x5FE2F79: UDT::epoll_wait2(int, int*, int*, int*, int*, long, int*, int*, int*, int*) (api.cpp:4277)
    by 0x5F0C626: gst_srt_object_read (gstsrtobject.c:1569)
    by 0x5F0F978: gst_srt_src_fill (gstsrtsrc.c:180)
    by 0x5F5A2A1: gst_base_src_default_create (gstbasesrc.c:1620)
    by 0x5F5C9AE: gst_base_src_get_range (gstbasesrc.c:2630)
    by 0x5F5EF5A: gst_base_src_loop (gstbasesrc.c:2959)
    by 0x4918B13: gst_task_func (gsttask.c:399)
    by 0x4A60B33: g_thread_pool_thread_proxy.lto_priv.0 (gthreadpool.c:354)
    by 0x4A5DC41: g_thread_proxy (gthread.c:826)
    by 0x4F532A4: start_thread (pthread_create.c:481)
    by 0x4C71322: clone (clone.S:95)

An issue has been started against libsrt here:

https://github.com/Haivision/srt/issues/2867

Part-of: !6080 (closed)