check: Add valgrind leak exception for srt test
The srt unittest test_src_listener_sink_caller will sometimes fail under valgrind with the following splat:
==17493== Memcheck, a memory error detector
==17493== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==17493== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==17493== Command: /builds/bilboed/gstreamer/build/subprojects/gst-plugins-bad/tests/check/elements_srt
==17493== Parent PID: 14579
==17493==
==17590==
==17590== HEAP SUMMARY:
==17590== in use at exit: 799,848 bytes in 2,182 blocks
==17590== total heap usage: 64,090 allocs, 61,908 frees, 37,891,032 bytes allocated
==17590==
==17590== 120 bytes in 1 blocks are definitely lost in loss record 1,563 of 1,681
==17590== at 0x4842FF5: operator new(unsigned long) (vg_replace_malloc.c:422)
==17590== by 0x6031E29: srt::sync::SetThreadLocalError(CUDTException const&) (sync_posix.cpp:461)
==17590== 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)
==17590== by 0x5FE2F79: UDT::epoll_wait2(int, int*, int*, int*, int*, long, int*, int*, int*, int*) (api.cpp:4277)
==17590== by 0x5F0C626: gst_srt_object_read (gstsrtobject.c:1569)
==17590== by 0x5F0F978: gst_srt_src_fill (gstsrtsrc.c:180)
==17590== by 0x5F5A2A1: gst_base_src_default_create (gstbasesrc.c:1620)
==17590== by 0x5F5C9AE: gst_base_src_get_range (gstbasesrc.c:2630)
==17590== by 0x5F5EF5A: gst_base_src_loop (gstbasesrc.c:2959)
==17590== by 0x4918B13: gst_task_func (gsttask.c:399)
==17590== by 0x4A60B33: g_thread_pool_thread_proxy.lto_priv.0 (gthreadpool.c:354)
==17590== by 0x4A5DC41: g_thread_proxy (gthread.c:826)
==17590== by 0x4F532A4: start_thread (pthread_create.c:481)
==17590== by 0x4C71322: clone (clone.S:95)See: https://gitlab.freedesktop.org/bilboed/gstreamer/-/jobs/54795504
An issue has been started against libsrt here: