Fix warnings in log while running tests.

When running tests sl4j would print warnings in the logs.
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Prevent this by adding slf4j-simple.jar in the test scope, to enable a proper logger for the tests.