allow suite.pm to tell mtr to skip the suite

mysql-test/README.suites

@@ -31,6 +31,8 @@ package that inherits from My::Suite.
 
 The suite.pm needs to have @ISA=qw(My::Suite) and it must end
 with bless {}; - that is it must return an object of that class.
+It can also return a string - in this case all tests in the suite
+will be skipped, with this string being printed as a reason.
 
 A suite class can define config_files() and servers() methods.
 

mysql-test/lib/mtr_cases.pm

@@ -997,6 +997,13 @@ sub collect_one_test_case {
     }
   }
 
+  if (not ref $::suites{$tinfo->{suite}})
+  {
+    $tinfo->{'skip'}= 1;
+    $tinfo->{'comment'}= $::suites{$tinfo->{suite}};
+    return $tinfo;
+  }
+
   # ----------------------------------------------------------------------
   # Append mysqld extra options to master and slave, as appropriate
   # ----------------------------------------------------------------------

mysql-test/suite/innodb_plugin/suite.pm

@@ -3,12 +3,14 @@ package My::Suite::InnoDB_plugin;
 @ISA = qw(My::Suite);
 
 ############# initialization ######################
-my @combinations=('none');
+my @combinations;
 
 push @combinations, 'innodb_plugin' if $ENV{HA_INNODB_PLUGIN_SO};
 push @combinations, 'xtradb_plugin' if $ENV{HA_XTRADB_SO};
 push @combinations, 'xtradb' if $::mysqld_variables{'innodb'} eq "ON";
 
+return "Neither innodb_plugin nor xtradb are available" unless @combinations;
+
 $ENV{INNODB_PLUGIN_COMBINATIONS}=join ':', @combinations
   unless $ENV{INNODB_PLUGIN_COMBINATIONS};