MDEV-33078 SysInfo.pm reports incorrect CPU count on macOS

When running on macOS, MTR will ask the operating system for the core count when --parallel=auto

mysql-test/lib/My/Platform.pm

@@ -23,7 +23,7 @@ use File::Path;
 use Carp;
 
 use base qw(Exporter);
-our @EXPORT= qw(IS_CYGWIN IS_MSYS IS_WINDOWS IS_WIN32PERL IS_AIX
+our @EXPORT= qw(IS_CYGWIN IS_MSYS IS_WINDOWS IS_WIN32PERL IS_AIX IS_MAC
 		native_path posix_path mixed_path
                 check_socket_path_length process_alive open_for_append);
 
@@ -70,6 +70,14 @@ BEGIN {
   }
 }
 
+BEGIN {
+  if ($^O eq "darwin") {
+    eval 'sub IS_MAC { 1 }';
+  }
+  else {
+    eval 'sub IS_MAC { 0 }';
+  }
+}
 
 #
 # native_path

mysql-test/mysql-test-run.pl

@@ -427,6 +427,10 @@ sub main {
     {
       $opt_parallel= $ENV{NUMBER_OF_PROCESSORS} || 1;
     }
+    elsif (IS_MAC)
+    {
+      $opt_parallel= `sysctl -n hw.ncpu`;
+    }
     else
     {
       my $sys_info= My::SysInfo->new();