fixed Too many connections error.

[ytetsuro]

I encountered error following.

A PHP Error was encountered

Severity:    Warning
Message:     mysqli::real_connect(): (08004/1040): Too many connections
Filename:    /vagrant/html/vendor/codeigniter/framework/system/database/drivers/mysqli/mysqli_driver.php
Line Number: 201

ci-phpunit-test is no close database connection, when call Loader::database second argument true.
for that reason encount error when frequent call Loader::database method in test case.

This problem solution is following.

• create collection class for db connection.
• collect db connection when call Loader::database second argument is true.
• close db connection from collected when call CIPHPUnitTestCase::tearDownAfterClass.

[coveralls]

Coverage decreased (-0.05%) to 72.757% when pulling ead6b48 on ytetsuro:feat/db-connection-close into e38a140 on kenjis:master.