improve robustness of web_seed unit tests

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9797 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged i2p_alert fix from RC_0_16

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9800 a83610d8-ad2a-0410-a6ab-fc0612d85776