Bug#49170 Inconsistent placement of semisync plugin prevents it from getting tested

Add $basedir/lib/plugin to the search paths for semisync plugins.

mysql-test/mysql-test-run.pl

@@ -1834,11 +1834,13 @@ sub environment_setup {
     my $lib_semisync_master_plugin=
       mtr_file_exists(vs_config_dirs('plugin/semisync',$semisync_master_filename),
 		      "$basedir/plugin/semisync/.libs/" . $semisync_master_filename,
-                      "$basedir/lib/mysql/plugin/" . $semisync_master_filename);
+                      "$basedir/lib/mysql/plugin/" . $semisync_master_filename,
+                      "$basedir/lib/plugin/" . $semisync_master_filename);
     my $lib_semisync_slave_plugin=
       mtr_file_exists(vs_config_dirs('plugin/semisync',$semisync_slave_filename),
 		      "$basedir/plugin/semisync/.libs/" . $semisync_slave_filename,
-                      "$basedir/lib/mysql/plugin/" . $semisync_slave_filename);
+                      "$basedir/lib/mysql/plugin/" . $semisync_slave_filename,
+                      "$basedir/lib/plugin/" . $semisync_slave_filename);
     if ($lib_semisync_master_plugin && $lib_semisync_slave_plugin)
     {
       $ENV{'SEMISYNC_MASTER_PLUGIN'}= basename($lib_semisync_master_plugin);