MDEV-32507 Spider: Use $MTR_SUITE_DIR for init-file files

This should fix certain CI builds where the spider suite test files
and the main suite test files do not follow the same relative paths
relations as the mariadb source.

storage/spider/mysql-test/spider/bugfix/t/mdev_27233.opt

---init-file=$MYSQL_TEST_DIR/../storage/spider/mysql-test/spider/bugfix/t/mdev_27233.sql
+--init-file=$MTR_SUITE_DIR/t/mdev_27233.sql

storage/spider/mysql-test/spider/bugfix/t/mdev_28218_init_file.opt

---init-file=$MYSQL_TEST_DIR/../storage/spider/mysql-test/spider/bugfix/t/mdev_28218_init_file.sql
+--init-file=$MTR_SUITE_DIR/t/mdev_28218_init_file.sql

storage/spider/mysql-test/spider/bugfix/t/mdev_28218_mixed.opt

 --plugin-load-add=ha_spider
---init-file=$MYSQL_TEST_DIR/../storage/spider/mysql-test/spider/bugfix/t/mdev_28218_mixed.sql
+--init-file=$MTR_SUITE_DIR/t/mdev_28218_mixed.sql

storage/spider/mysql-test/spider/bugfix/t/udf_mysql_func_early_init_file.opt

---init-file=$MYSQL_TEST_DIR/../storage/spider/mysql-test/spider/bugfix/t/udf_mysql_func_early_init_file.sql
+--init-file=$MTR_SUITE_DIR/t/udf_mysql_func_early_init_file.sql