Bug #54364 Allow multiple --experimental command line options for MTR

Bummer: MTR fails if a test fails and no --experimental is used
$experimental_test_cases must be intialized

mysql-test/mysql-test-run.pl

@@ -188,7 +188,7 @@ my $config; # The currently running config
 my $current_config_name; # The currently running config file template
 
 our @opt_experimentals;
-our $experimental_test_cases;
+our $experimental_test_cases= [];
 
 my $baseport;
 # $opt_build_thread may later be set from $opt_port_base