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: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2955)

util/TLSProxy/Proxy.pm

@@ -284,32 +284,30 @@ sub clientstart
 
     #Wait for either the server socket or the client socket to become readable
     my @ready;
-    while(!(TLSProxy::Message->end) && (@ready = $sel->can_read)) {
+    my $ctr = 0;
+    while(     (!(TLSProxy::Message->end)
+                || (defined $self->sessionfile()
+                    && (-s $self->sessionfile()) == 0))
+            && $ctr < 10
+            && (@ready = $sel->can_read(1))) {
         foreach my $hand (@ready) {
             if ($hand == $server_sock) {
                 $server_sock->sysread($indata, 16384) or goto END;
                 $indata = $self->process_packet(1, $indata);
                 $client_sock->syswrite($indata);
+                $ctr = 0;
             } elsif ($hand == $client_sock) {
                 $client_sock->sysread($indata, 16384) or goto END;
                 $indata = $self->process_packet(0, $indata);
                 $server_sock->syswrite($indata);
+                $ctr = 0;
             } else {
-                print "Err\n";
-                goto END;
+                $ctr++
             }
         }
     }
 
-    for (my $ctr = 0;
-         defined $self->sessionfile()
-            && (!(-f $self->sessionfile()) || $ctr == 3);
-         $ctr++) {
-        sleep 1;
-    }
-
-    die "Session file not created"
-        if (defined $self->sessionfile() && !(-f $self->sessionfile()));
+    die "No progress made" if $ctr >= 10;
 
     END:
     print "Connection closed\n";