Added explicit closing the sockets. To prevent messing with UDP sockets keeping ports busy.

[ethouris]

The reason why this is necessary is that it has been observed in some test CI environments that sometimes sockets fail to bind a port, for which the reason could be that the sockets may be deleted with too large delay and cause the next test to fail due to inability to bind the port.

[maxsharabayko]

This PR does not look clean. Instead of closing a socket in the testing function itself, a kind of a "garbage socket collector" is being created, that accumulates created sockets and closes them in the end. This code distracts and complicates the testing code.

For example, here the client_socket could just be closed in the end. The test uses ASSERT instead of EXPECT, so that once it fires, the function does not reach the end and the socket won't be closed. However, almost every test uses the TestInit class after PR #2681, which calls srt_cleanup() in its destructor. The srt-cleanup() function is expected to close all open sockets.

If a socket should be closed anyway, It may be cleaner to use some smart socket object, that would close the socket in its destructor, and can be converted to SRT_SOCKET when passed to SRT API functions directly.

class unique_srt_socket
{
public:
    unique_srt_socket(SRTSOCKET s);
   ~unique_srt_socket();
   // ...
};