Bug #42986 MTR2 should not allow 0 as a value for --parallel

Add sanity check that $opt_parallel > 0

mysql-test/mysql-test-run.pl

@@ -1075,6 +1075,14 @@ sub command_line_setup {
     $opt_shutdown_timeout= 0; # Kill processes instead of nice shutdown
   }
 
+  # --------------------------------------------------------------------------
+  # Check parallel value
+  # --------------------------------------------------------------------------
+  if ($opt_parallel < 1)
+  {
+    mtr_error("0 or negative parallel value makes no sense, use positive number");
+  }
+
   # --------------------------------------------------------------------------
   # Record flag
   # --------------------------------------------------------------------------