Fix race condition in TLSProxy

Normally TLSProxy waits for the s_server process to finish before
continuing. However in cases where serverconnects > 1 we need to keep the
s_server process around for a later test so we continue immediately. This
means that TAP test output can end up being printed to stdout at the same
time as s_server is printing stuff. This confuses the test runner and can
cause spurious test failures. This commit introduces a small delay in cases
where serverconnects > 1 in order to give s_server enough time to finish
what it was doing before we continue to the next test.

Fixes #4129

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4661)