public void setTestSQL(String testSQL) {
        this.testSQL = testSQL;
    }
Sets the SQL statement used to test if a connection is valid.  House keeping
and before/after connection tests make use of this.  This
should be something that causes the minimal amount of work by the database
server and is as quick as possible.

@param testSQL the SQL statement that will be run to test a connection.
xmppserver/src/main/java/org/jivesoftware/database/DefaultConnectionProvider.java