MDEV-17076: mtr max-{core,datadir} 0 means 0

There was no ability to set the mtr arguments of:
* --max-save-core; and
* --max-save-datadir

to 0. This is desireable in an automatied scenario where space
is limited hence targeting 10.1 branch.

We take away the 0 means unlimited aspect for these,
however, perl can handle some big numbers so they may as well be
close enough to unlimited for all meaningful purposes.

mysql-test/mysql-test-run.pl

@@ -697,8 +697,7 @@ ($$$)
                   My::CoreDump->show($core_file, $exe_mysqld, $opt_parallel);
 
                   # Limit number of core files saved
-                  if ($opt_max_save_core > 0 &&
-                      $num_saved_cores >= $opt_max_save_core)
+                  if ($num_saved_cores >= $opt_max_save_core)
                   {
                     mtr_report(" - deleting it, already saved",
                                "$opt_max_save_core");
@@ -714,8 +713,7 @@ ($$$)
             },
             $worker_savedir);
 
-	    if ($opt_max_save_datadir > 0 &&
-		$num_saved_datadir >= $opt_max_save_datadir)
+	    if ($num_saved_datadir >= $opt_max_save_datadir)
 	    {
 	      mtr_report(" - skipping '$worker_savedir/'");
 	      rmtree($worker_savedir);
@@ -6220,12 +6218,12 @@ ($)
                         test(s)
   max-save-core         Limit the number of core files saved (to avoid filling
                         up disks for heavily crashing server). Defaults to
-                        $opt_max_save_core, set to 0 for no limit. Set
-                        it's default with MTR_MAX_SAVE_CORE
+                        $opt_max_save_core. Set its default with
+                        MTR_MAX_SAVE_CORE
   max-save-datadir      Limit the number of datadir saved (to avoid filling
                         up disks for heavily crashing server). Defaults to
-                        $opt_max_save_datadir, set to 0 for no limit. Set
-                        it's default with MTR_MAX_SAVE_DATADIR
+                        $opt_max_save_datadir. Set its default with
+                        MTR_MAX_SAVE_DATADIR
   max-test-fail         Limit the number of test failures before aborting
                         the current test run. Defaults to
                         $opt_max_test_fail, set to 0 for no limit. Set