Move the InnoDB Plugin tests from storage/innodb_plugin/mysql-test/ where

they are ignored to a new test suite "innodb_plugin".

Remove a hack in mtr that was deployed to run the builtin InnoDB tests against
the InnoDB Plugin. Also detect if a test is an 'innodb plugin test' and if so
then transparently replace the builtin InnoDB with the InnoDB Plugin.

mysql-test/suite/innodb_plugin/t/innodb_bug41904.test

@@ -0,0 +1,14 @@
+#
+# Make sure http://bugs.mysql.com/41904 remains fixed.
+#
+
+-- source include/not_embedded.inc
+-- source include/have_innodb_plugin.inc
+
+CREATE TABLE bug41904 (id INT PRIMARY KEY, uniquecol CHAR(15)) ENGINE=InnoDB;
+
+INSERT INTO bug41904 VALUES (1,NULL), (2,NULL);
+
+CREATE UNIQUE INDEX ui ON bug41904 (uniquecol);
+
+DROP TABLE bug41904;