tls: fix segfault in pummel/test-tls-ci-reneg-attack

Commit 4e5fe2d4 changed the way how process.nextTick() works:

    process.nextTick(function foo() {
      process.nextTick(function bar() {
        // ...
      });
    });

Before said commit, foo() and bar() used to run on separate event loop ticks
but that is no longer the case.

However, that's exactly the behavior that the TLS renegotiation attack guard
relies on. It gets called by OpenSSL and needs to defer the 'error' event to a
later tick because the default action is to destroy the TLS context - the same
context that OpenSSL currently operates on.

When things change underneath your feet, bad things happen and OpenSSL is no
exception. Ergo, use setImmediate() instead of process.nextTick() to ensure
that the 'error' event is actually emitted at a later tick.

Fixes #3840.