Bug#40715 mtr fails when running the same test case in parallel threads

mysql-test/lib/My/Test.pm:
  Use a more unique key to identify which test has been serialized

mysql-test/lib/My/Test.pm

@@ -26,9 +26,7 @@ sub new {
 #
 sub key {
   my ($self)= @_;
-  my $key= $self->{name};
-  $key.= "+".$self->{combination} if $self->{combination};
-  return $key;
+  return $self->{key};
 }
 
 
@@ -57,6 +55,9 @@ sub is_failed {
 sub write_test {
   my ($test, $sock, $header)= @_;
 
+  # Give the test a unique key before serializing it
+  $test->{key}= "$test" unless defined $test->{key};
+
   print $sock $header, "\n";
   while ((my ($key, $value)) = each(%$test)) {
     print $sock  $key, "= ";