MDEV-32387 - prevent output during temporary changes to STDOUT/STDERR

create_process() temporarily changes STDOUT/STDERR output to error log file

This might redirect mtr output on Windows, so avoid it by holding
flush_lock.

mysql-test/lib/My/SafeProcess/Base.pm

@@ -114,6 +114,10 @@ sub create_process {
 
     lock($win32_spawn_lock);
 
+    # STDOUT/STDERR are temporarily reassigned, avoid prevent
+    # output from another thread
+    lock($mtr_report::flush_lock);
+
     #printf STDERR "stdin %d, stdout %d, stderr %d\n",
     #    fileno STDIN, fileno STDOUT, fileno STDERR;
 

mysql-test/lib/mtr_report.pm

@@ -88,7 +88,7 @@ sub flush_out {
 }
 
 use if $^O eq "MSWin32", "threads::shared";
-my $flush_lock :shared;
+our $flush_lock :shared;
 
 # Print to stdout
 sub print_out {