Windows SChannel SSL test failure

This problem started since PROTON-1048 which did not touch the Proton-C SChannel related code - it just tweaked the test infrastructure to use alternate certificate formats for Windows.

The proton_tests.ssl.SslTest.test_server_hostname_authentication test randomly crashes on Windows, presumably from some memory corruption somewhere. It often runs fine. Seems to happen more frequently on some systems than others. Not yet seen on a 64 bit build.