Fix a hang in tests that use sessionfile

The logic for testing whether the sessionfile has been created or not
was faulty and could result in race conditions. If you "lose" the tests
hang waiting for a session file that's never going to arrive.

Fixes #2950

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2955)