Fix a Proxy race condition

Issue #3562 describes a problem where a race condition can occur in the
Proxy such that a test "ok" line can appear in the middle of other text
causing the test harness to miss it. The issue is that we do not wait for
the client process to finish after the test is complete, so that process may
continue to write data to stdout/stderr at the same time that the test
harness does.

This commit fixes TLSProxy so that we always wait for the client process to
finish before continuing.

Fixes #3562

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3567)
(cherry picked from commit b72668a0)