tests: Fix segfault with mbedTLS built without threading support
torture_rand and torture_server_x11 call ssh_init without checking
the return value. If mbedTLS is built without threading support
ssh_init fails but the tests continue and then segfault since threading
wasn't correctly initialised.
Add a section that documents requirements for mbedTLS usage in a
multi threaded environment to README.mbedtls.
Signed-off-by: Juraj Vijtiuk <firstname.lastname@example.org>
Reviewed-by: Andreas Schneider <email@example.com>