Bug#59955 - engines/funcs/ps_string_not_null test needs better cleanup

Correcting clean up command at the start of test.

mysql-test/suite/engines/funcs/t/ps_string_not_null.test

 --disable_warnings
-DROP TABLE IF EXISTS t2;
+DROP TABLE IF EXISTS t1;
 --enable_warnings
 CREATE TABLE t1(c1 CHAR(100) NOT NULL);
 PREPARE stmt1 FROM 'INSERT INTO t1 (c1) VALUES(?)';