Increase test coverage

As reported during the vote thread [1], and also in another separate thread [2], it would be good if we could increase the test coverage of Apache Commons DBCP.

Issue assigned to me, but all welcome to contribute.

[1] https://markmail.org/message/nvckwmylpqlrf3kg

[2] https://markmail.org/message/ehnkrwfgup2tuqa4