TLSProxy/Proxy.pm: preclude output intermix.

s_server -rev emits info output on stderr, i.e. unbufferred, which
risks intermixing with output from TLSProxy itself on non-line
boundaries, which in turn is confusing to TAP parser.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5975)

util/perl/TLSProxy/Proxy.pm

@@ -239,7 +239,7 @@ sub start
     open(my $savedin, "<&STDIN");
 
     # Temporarily replace STDIN so that sink process can inherit it...
-    $pid = open(STDIN, "$execcmd |") or die "Failed to $execcmd: $!\n";
+    $pid = open(STDIN, "$execcmd 2>&1 |") or die "Failed to $execcmd: $!\n";
     $self->{real_serverpid} = $pid;
 
     # Process the output from s_server until we find the ACCEPT line, which