Logger interface compatible with testing package

Issue description

It would be convenient if the Logger interface was compatible with the testing logger (Logf).

Example code

func TestMyTest(t *testing.T) {
// start mysql in a test with a container for instance
// test polls mysql waiting for the connection to come up
// meanwhile go-sql-driver/mysql dumps errors into the testing package
  someCodeSucceeds
// testing does not report log statements
}

Error log

mysql dumps messages into the terminal regardless of testing success or failure
[mysql] 2017/05/16 15:46:20 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:20 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:20 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:21 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:21 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:21 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:22 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:22 packets.go:33: unexpected EOF
[mysql] 2017/05/16 15:46:22 packets.go:33: unexpected EOF

Configuration

Driver version (or git SHA): 1.3.0 Go version: run go version in your console 1.8.1 Server version: E.g. MySQL 5.6, MariaDB 10.0.20 MySQL 5.6 Server OS: E.g. Debian 8.1 (Jessie), Windows 10 Oracle Linux 7